@pulo97 This feature broke a few versions back and it's been annoying me for long enough to figure out a work around, though it's kind of a pain and it only works 1 as I'll describe below.
Bug Add your wallpaper does not set the current wallpaper.
Current behavior: The open button has no effect.
Repro steps:
Right click the current wallpaper and click the Add your wallpaper button and select an image.
Expected behavior: The current wallpaper is set to the image and the image added to the Recent Images queue.
Workaround:
Disclaimer, this is essentially brute force but worked and I'm not getting paid to do more refined test work.
I located an older build of the product from one of many caches you can find on the web by searching, probably github. I selected a build and chose Opera_58.0.3135.90_Setup_x64.exe
I uninstalled Opera then ran the executable. On first launch you can set wallpapers to your hearts content - once. I selected a bunch which now appear in the Recent gallery and so can be reused.
So what does Opera do? It upgrades itself in the background thus bringing back the bug; as soon as you exit this session the feature will be broken again by which I mean it won't allow you to set a new wallpaper, only from your Recent.
Second workaround: transferring those wallpapers to another computer without having to do it all again which is tedious. So I left Opera install and copied the appdata settings (low and roaming) from one to the other. (I actually cleaned out stuff like log files first). It had the desired effect: the second computer can now use the images from recent. Note: The file paths on the two machines are identical. I didn't determine if they are using encoded paths or simply making copies of the image but I believe it's the latter.
Maybe this will help get a fix in the next build, it's likely to be minor fix in my opinion, but I haven't look at code base.
Still my browser of choice on Windows (and Android where this issue doesn't apply)