Yeah.. it's only really available on older images, where it's had time to see what it thinks are similar.
Have a look at woberto's link above, and then refresh that image until the similar images box comes up on the left hand side, you'll see what I mean.
Maybe we need to expand on that. There was a reason it didn't work for new images. It's just in the 5 years since we put it there I don't remember what it is
I'd like to look at developing some new features on the sites again.
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