I might be more concerned about false negative matches, but it depends upon what the OP meant by comparing images. You could upload a jpeg of a dog that gets an MD5sum of 00000 (or whatever). I could upload a png of that same photo and that gets an M5sum of ABDEF. Someone else could come along an upload another jpeg at a slightly different compression setting and get an MD5sum of A188F.
They're all different files, but they all represent the same image. To try and compensate for that, it'd require a more advanced technique like one of the articles linked to later on here in the thread to actually compare upon how the image actually looks, not merely how it's stored.
Of course, if you just want to make sure the same file isn't uploaded twice, the md5sum should do just fine.
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