CoreImage quick'n'dirty sample: Contrast/Saturation/Brightness editing (screenshot http://twitpic.com/93gfty)

AppDelegate.cs

using System;
using System.Collections.Generic;
using System.Linq;

using MonoTouch.Foundation;
using MonoTouch.UIKit;

namespace ColorControl {
	// The UIApplicationDelegate for the application. This class is responsible for launching the 
	// User Interface of the application, as well as listening (and optionally responding) to 
	// application events from iOS.
	[Register ("AppDelegate")]
	public partial class AppDelegate : UIApplicationDelegate {

		static void Main (string[] args)
		{
			// if you want to use a different Application Delegate class from "AppDelegate"
			// you can specify it here.
			UIApplication.Main (args, null, "AppDelegate");
		}

		// class-level declarations
		UIWindow window;
		UINavigationController navigationController;
		UIViewController viewController;

		//
		// This method is invoked when the application has loaded and is ready to run. In this 
		// method you should instantiate the window, load the UI into it and then make the window
		// visible.
		//
		// You have 17 seconds to return from this method, or iOS will terminate your application.
		//
		public override bool FinishedLaunching (UIApplication app, NSDictionary options)
		{
			// create a new window instance based on the screen size
			window = new UIWindow (UIScreen.MainScreen.Bounds);
			
			viewController = new ImageViewController();
			
			navigationController = new UINavigationController();
			navigationController.PushViewController (viewController, false);

			// If you have defined a view, add it here:
			window.AddSubview (navigationController.View);
			
			// make the window visible
			window.MakeKeyAndVisible ();
			
			return true;
		}
	}
}

Camera.cs

//
// Camera.cs: Support code for taking pictures
//
// Copyright 2010 Miguel de Icaza
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
// 
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
// 
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.

using System;
using MonoTouch.UIKit;
using MonoTouch.Foundation;

namespace TweetStation
{
	//
	// A static class that will reuse the UIImagePickerController
	// as iPhoneOS has a crash if multiple UIImagePickerController are created
	//   http://stackoverflow.com/questions/487173
	// (Follow the links)
	//
	public static class Camera
	{
		static UIImagePickerController picker;
		static Action<NSDictionary> _callback;
		
		static void Init ()
		{
			if (picker != null)
				return;
			
			picker = new UIImagePickerController ();
			picker.Delegate = new CameraDelegate ();
		}
		
		class CameraDelegate : UIImagePickerControllerDelegate {
			public override void FinishedPickingMedia (UIImagePickerController picker, NSDictionary info)
			{
				var cb = _callback;
				_callback = null;
				
				picker.DismissModalViewControllerAnimated (true);
				cb (info);
			}
		}
		
		public static void TakePicture (UIViewController parent, Action<NSDictionary> callback)
		{
			Init ();
			picker.SourceType = UIImagePickerControllerSourceType.Camera;
			_callback = callback;
			parent.PresentModalViewController (picker, true);
		}
		
		public static void SelectPicture (UIViewController parent, Action<NSDictionary> callback)
		{
			Init ();
			picker.SourceType = UIImagePickerControllerSourceType.PhotoLibrary;
			_callback = callback;
			parent.PresentModalViewController (picker, true);
		}
	}
}

ImageViewController.cs

using System;
using System.Drawing;
using MonoTouch.AssetsLibrary;
using MonoTouch.UIKit;
using MonoTouch.Foundation;
using MonoTouch.CoreImage;
using MonoTouch.CoreGraphics;

namespace ColorControl {

	public class ImageViewController : UIViewController {
		
		UIImage sourceImage, displayImage;
		NSDictionary sourceMeta;

		UIButton cameraButton, saveButton, resetButton;
		UIImageView imageView;
		UISlider sliderC, sliderS, sliderB;
		UILabel labelC, labelS, labelB;

		public override void ViewDidLoad ()
		{
			base.ViewDidLoad ();
			Title = "Color Controls";
			View.BackgroundColor = UIColor.White;

			cameraButton = UIButton.FromType (UIButtonType.RoundedRect);
			cameraButton.Frame = new RectangleF(10, 10, 90,40);
			cameraButton.SetTitle ("Camera", UIControlState.Normal);
			cameraButton.TouchUpInside += (sender, e) => {
			
				TweetStation.Camera.TakePicture (this, (obj) =>{
					// https://developer.apple.com/library/ios/#documentation/uikit/reference/UIImagePickerControllerDelegate_Protocol/UIImagePickerControllerDelegate/UIImagePickerControllerDelegate.html#//apple_ref/occ/intfm/UIImagePickerControllerDelegate/imagePickerController:didFinishPickingMediaWithInfo:
					var photo = obj.ValueForKey(new NSString("UIImagePickerControllerOriginalImage")) as UIImage;
					var meta = obj.ValueForKey(new NSString("UIImagePickerControllerMediaMetadata")) as NSDictionary;
			
					sourceImage = photo;
					displayImage = sourceImage.Scale(new SizeF(300, 200));
					imageView.Image = displayImage;
					sourceMeta = meta;
				});
			};
			View.Add (cameraButton);

			resetButton = UIButton.FromType (UIButtonType.RoundedRect);
			resetButton.Frame = new RectangleF(110, 10, 90, 40);
			resetButton.SetTitle ("Reset", UIControlState.Normal);
			resetButton.TouchUpInside += (sender, e) => {
				sliderS.Value = 1;	
				sliderB.Value = 0;
				sliderC.Value = 1;
				HandleValueChanged (sender, e);
			};
			View.Add (resetButton);

			saveButton = UIButton.FromType (UIButtonType.RoundedRect);
			saveButton.Frame = new RectangleF(210, 10, 90, 40);
			saveButton.SetTitle ("Save", UIControlState.Normal);
			saveButton.TouchUpInside += (sender, e) => {
				ALAssetsLibrary library = new ALAssetsLibrary();
				
				var img = AdjustImage (sourceImage);
				
				if(sourceMeta == null) sourceMeta = new NSDictionary();

				library.WriteImageToSavedPhotosAlbum (img.CGImage, sourceMeta, (assetUrl, error) => {
					Console.WriteLine ("SAVED TO assetUrl:"+assetUrl);
					new UIAlertView("Saved", "Photo saved to Camera Roll", null, "OK", null).Show ();
				});
			};
			View.Add (saveButton);	

			labelC = new UILabel(new RectangleF(10, 70, 90, 20));
			labelS = new UILabel(new RectangleF(10, 110, 90, 20));
			labelB = new UILabel(new RectangleF(10, 150, 90, 20));

			labelC.Text = "Contrast";
			labelS.Text = "Saturation";
			labelB.Text = "Brightness";

			View.Add (labelC);
			View.Add (labelS);
			View.Add (labelB);

			sliderB = new UISlider(new RectangleF(100,  70, 210, 20));
			sliderS = new UISlider(new RectangleF(100, 110, 210, 20));
			sliderC = new UISlider(new RectangleF(100, 150, 210, 20));
			
// http://developer.apple.com/library/mac/#documentation/graphicsimaging/reference/CoreImageFilterReference/Reference/reference.html#//apple_ref/doc/filter/ci/CIColorControls

			sliderS.MinValue = 0;
			sliderS.MaxValue = 2;
			sliderS.Value = 1;
			
			sliderB.MinValue = -1;
			sliderB.MaxValue = 1;
			sliderB.Value = 0;

			sliderC.MinValue = 0;
			sliderC.MaxValue = 4;
			sliderC.Value = 1;
			
			sliderC.TouchUpInside += HandleValueChanged;
			sliderS.TouchUpInside += HandleValueChanged;
			sliderB.TouchUpInside += HandleValueChanged;
			
			View.Add (sliderC);
			View.Add (sliderS);
			View.Add (sliderB);

			imageView = new UIImageView(new RectangleF(10, 190, 300, 200));
			sourceImage = UIImage.FromFile ("clouds.jpg");
			displayImage = sourceImage;
			imageView.Image = displayImage;
			View.Add (imageView);

			if (!UIImagePickerController.IsSourceTypeAvailable (UIImagePickerControllerSourceType.Camera)) {
				cameraButton.SetTitle ("No camera", UIControlState.Disabled);
				cameraButton.SetTitleColor (UIColor.Gray, UIControlState.Disabled);
				cameraButton.Enabled = false;
			}
		}

		void HandleValueChanged (object sender, EventArgs e)
		{	// use the low-res version
			imageView.Image = AdjustImage (displayImage);
		}
		CIColorControls colorCtrls;
		UIImage AdjustImage (UIImage image) {
			if (colorCtrls == null)
				colorCtrls = new CIColorControls () {
					Image = CIImage.FromCGImage (image.CGImage),
					Brightness = sliderB.Value, 
					Saturation = sliderS.Value, 
					Contrast = sliderC.Value
				};
			else {
				colorCtrls.Brightness = sliderB.Value; 
				colorCtrls.Saturation = sliderS.Value; 
				colorCtrls.Contrast = sliderC.Value;
				colorCtrls.Image = CIImage.FromCGImage(image.CGImage);
			}

			var output = colorCtrls.OutputImage;
			var context = CIContext.FromOptions (null);
			var result = context.CreateCGImage (output, output.Extent);
			colorCtrls = null;

			return UIImage.FromImage(result);
		}
	}
}