Looks like those are just upscaled versions of this. Probably one site upscaled it to HD and others took it from that one. I think those sites usually steal pics form each other, but they also do browse DA, Flickr and Pixiv. And probably ask their own users to upload any good pictures they find online.
I'm lucky, because I still use 1280x1024 monitor, so original pic is big enough for me.
Yeah I figured that also. Well at one point I tried to contact those sites, however it's rather impossible to get one image off from there. It makes me sad when people don't give credits. I have recently uploaded smaller versions of my images to prevent this problem..