How can I compare two images?

I have a list object that contains urls to images that exist on the internet. In my game, I need to compare these images to the images that exist in a user-selected folder on the user's home computer.

What would be the fastest way to create a new list in-game containing only the urls to the images that exist in the user-selected folder?

If I'm not making sense here, please ask me to clarify. Basically, I need a group of events that can compare an image online to an image on the user's home computer, pixel-by-pixel. Any help you can give me is much appreciated. :)

Thanks