In the future, I want to draw and design background images for the app. I plan to create them under a CC0 license so that developers can use them without bureaucratic complications. However, I decided to clarify in advance what criteria they must meet to be accepted into the default set of wallpapers.
Or are there no plans to expand the built-in collection of backgrounds in the near future? 
no immediate plans. First idea was to rotate them with each major release, but we only changed them once and then kept them the same afterwards.
We still have some unused pictures in interface/raw-impressions at main · deltachat/interface · GitHub and the current default wallpaper/doodle background is in interface/background at main · deltachat/interface · GitHub.
Mobile doesn’t have a multiple default images at this point, I think it would be nice to haven, but was and is not a priority so far.
We have to consider app size, so we can not add too many extra files. But I think if you or others come up with some good ones we could add a few more to desktop (but not too many, I personally think it should not be more than 12 or 16, we can rotate them, like replace old ones if we hit the limit).
you could submit them by posting them here or as a pr to the https://github.com/deltachat/interface repo.
But I’m not promising anything here, just sharing my opinion, this would need to be discussed with other team members first.
By the way, in DC-Desktop it is possible to use GIFs as wallpaper. I don’t know of any other messenger that can do this.
