With this category of `UIImageView` you can apply a mask (grey-scale jpg) onto a given image. The mask is then applied on that existing image. With this technique you can use 2 compressed JPG's (not transparent image + mask) rather than one big PNG file.

UIImageView+Mask.h

#import <Foundation/Foundation.h>

@interface UIImageView (Mask)

- (void)applyMask:(NSString *)maskName;

@end

UIImageView+Mask.m

#import "UIImageView+Mask.h"
#import "UIImage+ImageLoading.h"


@implementation UIImageView (Mask)

- (void)applyMask:(NSString *)maskName
{
	CGImageRef imageRef = self.image.CGImage;
	CGImageRef maskRef = [UIImage imageNamedUncached:maskName].CGImage;

	CGImageRef mask = [self getMask:maskRef];
	CGImageRef imageWithAlpha= [self convertToAlphaImage:imageRef];

	CGImageRef finalImageRef = CGImageCreateWithMask(imageWithAlpha, mask);

	self.image = [UIImage imageWithCGImage:finalImageRef];
}

- (CGImageRef)getMask:(CGImageRef)imageRef
{
	return CGImageMaskCreate(CGImageGetWidth(imageRef),
							 CGImageGetHeight(imageRef),
							 CGImageGetBitsPerComponent(imageRef),
							 CGImageGetBitsPerPixel(imageRef),
							 CGImageGetBytesPerRow(imageRef),
							 CGImageGetDataProvider(imageRef), nil, YES);
}

- (CGImageRef)convertToAlphaImage:(CGImageRef)imageRef
{
	CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();

	CGFloat width = CGImageGetWidth(imageRef);
	CGFloat height = CGImageGetWidth(imageRef);

	CGContextRef contextWithAlpha = CGBitmapContextCreate(nil, (size_t) width, (size_t) height, 8, (size_t) (4*width), colorSpace, kCGImageAlphaPremultipliedFirst);
	CGContextDrawImage(contextWithAlpha, CGRectMake(0, 0, width, height), imageRef);
	CGImageRef imageWithAlpha = CGBitmapContextCreateImage(contextWithAlpha);

	CGColorSpaceRelease(colorSpace);
	CGContextRelease(contextWithAlpha);

	return imageWithAlpha;
}

@end