License of PNG/JPEG backgrounds?

As far as I can see, the license(s) of JPEG and PNG backgrounds in this repo isn't specified anywhere. For the SVG backgrounds, I can see by reading the SVG files themselves that they are licensed under CC BY SA 4.0, e.g. here. However, I don't see anything comparable for the non-SVG backgrounds. Can this point be clarified and documented?

added 7. Licensing label

added 5. Background label

Unfortunately I really have no idea. @alexxcons, you've been involved in the backdrop contests, yeah? Was there ever a license that we required the backdrops be released to us under?

Looks like the PNG backgrounds were added by @ochosi ... if Simon is still checking gitlab mail, maybe he knows. Might also be worth going through mailing list archives to figure this out, maybe there's something there.

For the old backgrounds, I have no idea. Maybe just the GPL2 applies, since it is part of the project?

IIRC knome designed the wallpaper for Xfce4.16 ... he idles in the xfce-dev matrix channel, will try to ping him.

For the new ones, CC BY-SA 4.0 was explicitly requested during the contest: artwork/public#1 (closed)

closed with commit 6b8c2dd2

This should be mostly cleared up now. We still are not sure where xfce-blue came from, but the rest are all CC BY-SA-4.0. @knome replaced the PNG versions of his backgrounds with SVG versions (!124 (merged)), clarifying the license, and I've added a README with more information about each background.

I'm not really sure what to do about xfce-blue. I think it's safe to assume that it was contributed under a permissive (probably copyleft) license compatible with xfdesktop's, but it's unclear who created it (@ochosi added it to git, at least), and we don't have explicit licensing for it. Downstream distributors can feel free to patch it out if they're uncomfortable with that. I suppose we could also remove it (it's 12 years old), but I'd prefer not to...