Automator: How to check if an image exists before downloading

I have an Automator workflow working correctly that:

1. Get Specified URLs (Media RSS feed with images)
2. Get Image URLs from Articles (Get URLs of images linked from teh articles)
3. Download URLs (to a folder on my desktop)

This is working great, though it redownloads every image each time I run the workflow. Is there a way that I can have Automator check to see if the image exists in the folder prior to download and only downloading if it doesn't? This would save a lot of time and be much more efficient for me.

Any help would be greatly appreciated!

Thanks,

-Dan