Helps resize a control to fill its parent. Works only with a Image control but may be enhanced to work with other controls. Works in Windows Phone, may work in Silverlight/WPF. See http://stackoverflow.com/questions/13535022/windows-8-image-uniformfill-centered

FillParentBehavior.cs

namespace MyPhoneApp.Behaviors
{
    using System;
    using System.Collections.Generic;
    using System.Linq;
    using System.Text;
    using System.Threading.Tasks;
    using System.Windows;
    using System.Windows.Controls;
    using System.Windows.Interactivity;
    using System.Windows.Media;
    using System.Windows.Media.Imaging;

    /// <summary>
    /// Helps resize a control to fill its parent.
    /// Provides a Stretch=UniformToFill behavior.
    /// </summary>
    public class FillParentBehavior : Behavior<FrameworkElement>
    {
        /// <summary>
        /// The parent element to measure.
        /// </summary>
        private FrameworkElement parent;

        /// <summary>
        /// Called after the behavior is attached to an AssociatedObject.
        /// </summary>
        /// <remarks>
        /// Override this to hook up functionality to the AssociatedObject.
        /// </remarks>
        protected override void OnAttached()
        {
            base.OnAttached();

            this.parent = VisualTreeHelper.GetParent(this.AssociatedObject) as FrameworkElement;
            if (this.parent != null)
            {
                this.parent.SizeChanged += this.OnParentSizeChanged;
                this.OnParentSizeChanged(null, null);
            }
        }

        /// <summary>
        /// Called when the behavior is being detached from its AssociatedObject, but before it has actually occurred.
        /// </summary>
        /// <remarks>
        /// Override this to unhook functionality from the AssociatedObject.
        /// </remarks>
        protected override void OnDetaching()
        {
            base.OnDetaching();

            if (this.parent != null)
            {
                this.parent.SizeChanged -= this.OnParentSizeChanged;
            }
        }

        /// <summary>
        /// Called when [parent size changed].
        /// </summary>
        /// <param name="sender">The sender.</param>
        /// <param name="e">The <see cref="SizeChangedEventArgs"/> instance containing the event data.</param>
        private void OnParentSizeChanged(object sender, SizeChangedEventArgs e)
        {
            double width = this.AssociatedObject.Width;
            double height = this.AssociatedObject.Height;
            var parentSize = new Size(this.parent.ActualWidth, this.parent.ActualHeight);
            var parentRatio = parentSize.Width / parentSize.Height;

            // determine optimal size
            if (this.AssociatedObject is Image)
            {
                var image = (Image)this.AssociatedObject;
                if (image.Source is BitmapImage)
                {
                    var bitmap = (BitmapImage)image.Source;
                    var imageSize = new Size(bitmap.PixelWidth, bitmap.PixelHeight);
                    var imageRatio = imageSize.Width / imageSize.Height;
                    if (parentRatio <= imageRatio)
                    {
                        // picture has a greater width than necessary
                        // use its height
                        width = double.NaN;
                        height = parentSize.Height;
                    }
                    else
                    {
                        // picture has a greater height than necessary
                        // use its width
                        width = parentSize.Width;
                        height = double.NaN;
                    }
                }
            }

            this.AssociatedObject.Width = width;
            this.AssociatedObject.Height = height;
        }
    }
}