Can you post links to some images you recognize from elsewhere? This should help figure out who they are getting their content from.
Good idea, some results are below. Almost all of the images I could find are at 123RF and / or CanStockPhoto. I also found
assetdelivery.com, which gives this message: "123RF image caching server".
Both assetdelivery.com and 123rf.com are registered to Andy Sitt, so it looks like a 123RF partner site.
What made me suspicious was that I found an image at ooopic that I only uploaded to Fotolia, but it was
one that was stolen by someone and uploaded to many sites, including 123RF. That's probably how it got there, I can't find it anymore.
But they must be partnering with more sites, maybe CanStockPhoto, since they have images I don't have at 123RF.
I guess it's explained then, thanks for the suggestion sadstock

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ooopicCanStockPhotoGraphic Leftovers123RFooopicDreamstimeooopicShutterStock (and at 123RF too)
ooopicCutcaster Also at 123RF, CanStockPhoto and Dreamstime