I ran past duplicate file searches not long ago with fdupes; here’s one specifically aimed at duplicate images. …
The aptly-named findimagedupes. Hey, that’s what I would name it, if it was my program. 😉
This one has a little history to it, so I’m a little surprised that it took me this long to find it. It’s the story of my life, really. 🙄
As I understand it, findimagedupes scans md5sums and “fingerprints” to find images that are the same, or visually similar.
One thing in particular I liked about it was the option to spawn an image viewer program to check the lists it produced.
I should say that I used the term “visually similar” above, because if I read the results right, I’m getting false positives on some matches on my system.
I stacked the deck with three straightaway copied documents in that folder, but it worries me a little that some of the others listed were not … quite the same.
Try it and see if you’re getting the same results. Not that it would matter much if it was designed to look for similar images, only if it was picking out files that were completely unalike. 😯
Inconsolation 
Reading the algorithm described on the manpage, I’m not surprised that there will be a lot of false positives after all that downsizing and blurring to generate the “fingerprints”. I also wouldn’t expect this program to detect images which have been heavily cropped. But it should work fairly well with files which have only been renamed, lightly cropped or resized.