My theory is that some unscrupulous company has become a partner with one (or maybe more) of the sites. They have hi rez versions of the images. They offer the images on a "wallpaper" site that allows free downloading of the image. The images get downloaded and posted to another "wallpaper" site. "Wallpaper" in this post meaning FREE. It spreads like wildfire and pretty soon the whole rest of the world has free images.
I think this because I found the same thing, only with Russian "wallpaper" sites. My image was being offered for free download at high resolution on one site. While checking out the links, I noticed a ton more "wallpaper" sites also in Russian.
And I've done a search and found a ton of Chinese "wallpaper" sites, too.
I don't understand why they offer the images for free. I presume that they are making a ton of money from all the advertising on the site. So maybe the offering of the images for free is kind of a "screw you" and they are willing to make the money off the ads.
We're all on the same side of the fence but a few things have to be corrected:
The partner sites don't automatically get all high res files once they are affiliated. Only in the event of a purchase the stock agency will transfer the high res file. That's what I have been told by numerous agencies.
The affiliates are not supposed to save the high res files or at least not host them on their servers for future purchases as it would make sales tracking impossible. Naturally the agencies have a high interest that their affiliates report proper sales figures so they can match those up with their transferred files. I would even go that far, that agencies perform "test purchases" to see if the affiliate reports it correctly. But this is speculation.
There are wallpaper sites and there are wallpaper sites.
The one in question here is literally selling printed wallpapers. They need to have an EL at least and they could do that with a proper license (I'm not saying that they do or don't have that).
Then there are the desktop wallpaper sites that indeed just use awesome images to drive traffic to their site for ad revenue. As soon as something is offered free it's about generating traffic, that's even the same with the stock agencies offering freebies. Those wallpaper sites simply don't have the licenses.
This would be another interesting task to find out if there was ever a wallpaper site that did purchase an EL or any other appropriate license for redistribution.
