This is a simple version of UrlBitmapDrawable, an Android Drawable that can automatically reload a URL from an image library.

UrlBitmapDrawable.java

/**
 * This is a version of BitmapDrawable that's associated with a particular URL.
 * It automatically caches and reloads itself as its Bitmap is flushed from memory.
 * 
 * When using this with an ImageView, use loadImageView(), as there is some special
 * handling needed when using ImageViews.
 */
public class UrlBitmapDrawable extends BitmapDrawable {

  private String mUrl;
	private Bitmap mDefaultBitmap;
	private WeakReference<ImageView> mImageView;

	private boolean mRetrieving;
	private boolean mFailed;

	public UrlBitmapDrawable(Resources resources, String url, Bitmap defaultBitmap) {
		super(resources, defaultBitmap);

		mUrl = url;
		mDefaultBitmap = defaultBitmap;

		// Start loading immediately
		retrieveImage(false);
	}

	/*
	 * We need to do perform a hack to work properly with ImageViews,
	 * so most of the time you will be using this method.
	 */
	public static UrlBitmapDrawable loadImageView(String url, ImageView imageView, Bitmap defaultBitmap) {
		UrlBitmapDrawable drawable = new UrlBitmapDrawable(imageView.getContext().getResources(), url, defaultBitmap);

		// If there's already a UrlBitmapDrawable associated with this ImageView,
		// disassociate it (so we don't have two UrlBitmapDrawables trying to
		// operate on the same ImageView).
		Drawable currDrawable = imageView.getDrawable();
		if (currDrawable instanceof UrlBitmapDrawable) {
			((UrlBitmapDrawable) currDrawable).mImageView = null;
		}

		imageView.setImageDrawable(drawable);
		drawable.mImageView = new WeakReference<ImageView>(imageView);

		return drawable;
	}

	@Override
	public void draw(Canvas canvas) {
		// If URL not set, or we could not retrieve the image for this drawable, don't draw (or draw default)
		if (mFailed) {
			if (mDefaultBitmap != null) {
				super.draw(canvas);
			}
			return;
		}

		// If URL not loaded (either being null or recycled), load it now (and fallback to default)
		Bitmap bitmap = getBitmap();
		if (bitmap == null || bitmap.isRecycled()) {
			if (mDefaultBitmap != null) {
				setBitmap(mDefaultBitmap);
			}
			retrieveImage(false);
		}
		else {
			super.draw(canvas);
		}
	}

	private void retrieveImage(boolean forceRetrieve) {
		if (!mRetrieving || forceRetrieve) {
			mRetrieving = true;

			MyImageLib.loadImage(mUrl, mCallbacks);
		}
	}

	private MyImageCallbacks mCallbacks = new MyImageCallbacks() {
		@Override
		public void onLoad(Bitmap bitmap) {
			if (bitmap != null) {
				setBitmap(bitmap);

				// This is a hack to get the ImageView to re-measure the
				// width/height of this Drawable without having to completely
				// re-implement ImageView.  Without a re-measure this drawable
				// can end up skewed in the ImageView.
				if (mImageView != null) {
					final ImageView imageView = mImageView.get();
					if (imageView != null) {
						imageView.setImageDrawable(null);
						imageView.setImageDrawable(UrlBitmapDrawable.this);
					}
				}
			}
			else {
				mFailed = true;
			}

			mRetrieving = false;
		}
	};

	//////////////////////////////////////////////////////////////////////////
	// These are stand-ins for an actual image loading library

	public static interface MyImageCallbacks {
		public void onLoad(Bitmap bitmap);
	}

	private static class MyImageLib {
		private static void loadImage(String url, MyImageCallbacks callbacks) {
			// Implement an image loading library here
		}
	}
}

Just a heads up, it looks like setBitmap is now private in BitmapDrawable. Any idea how to get around that?