Usage as follows:

public class ImageManager : IUrlImageUpdated
{
	public delegate void ImageLoadedDelegate(string id, UIImage image);
	public event ImageLoadedDelegate ImageLoaded;

	UrlImageStore imageStore;
	
    private ImageManager()
    {
		imageStore = new UrlImageStore ("myImageStore", processImage);						            
    }
					
	private static ImageManager instance;
	
	public static ImageManager Instance
	{
		get
		{
			if (instance == null)
				instance = new ImageManager ();
			
			return instance;
		}	
	}
	
	// this is the actual entrypoint you call
	public UIImage GetImage(string imageUrl)
	{
		return imageStore.RequestImage (imageUrl, imageUrl, this);
	}
		
	public void UrlImageUpdated (string id, UIImage image)
	{
		// just propagate to upper level
		if (this.ImageLoaded != null)
			this.ImageLoaded(id, image);
	}
	
	// This handles our ProcessImageDelegate
	// just a simple way for us to be able to do whatever we want to our image
	// before it gets cached, so here's where you want to resize, etc.
	UIImage processImage(string id, UIImage image)
	{
		return image;
	}		
}

public class MyUIViewController : UIViewController
{
    // in some UIViewController simply request the image from the manager and register for the callback delegate to update then
    // so lets assume we have view controller and it contains imageView instance of UIImageView
    public override ViewDidLoad ()
    {
		// get the image by some URL
		UIImage image = ImageManager.Instance.GetProductImage (_imageUrl);	
		
		if (image == null) // it is not available cached, so we will wait for it
		{
			// register for callback
			ImageManager.Instance.ImageLoaded += HandleImageManagerInstanceImageLoaded;
		}
		else
		{
			// it exists so show it here
			if (imageView != null)
				imageView.Image = image;
			
			if (activityIndicator != null)
				activityIndicator.StopAnimating();
		}
    }
    
    void HandleImageManagerInstanceImageLoaded (string id, UIImage image)
	{
		if (id == _imageUrl)
		{
			// deregister the handler
			ImageManager.Instance.ImageLoaded -= HandleImageManagerInstanceImageLoaded;
			
			this.InvokeOnMainThread (delegate {
				
				if (_imageView != null)
					_imageView.Image = image;
				
				if (_activityIndicator != null)
					_activityIndicator.StopAnimating();
			});
		}
	}
}