Image comparison

Hi. I have a C# program I made to simplify browsing image repository websites. It's a client that uses their API to pull down thumbnails which I browse then if I like what I see, I click a thumbnail and it adds it to the download queue. One thing I'd like to implement is the ability for it to process the queue and compare the images to see if there are dups. Sometimes there may be multiple versions of a photo like one with glasses, one without glasses, one with the hair recolored, etc. I'd like the code I purchase to accept a collection of images then return me some data letting me know which of those meet a threshold of similarity so I can show a popup dialog displaying the similar images and asking the user which to keep.