I'm not sure if I understand your question correctly: you are trying to change the screen background, i.e. the desktop wallpaper from within your browser? If so, then that is of course not possible (and that's a good thing, believe me, because if it was, then it is just a matter of time until advertisers discover this!) but of course some browsers allow to right-click any image and set it as desktop background. Not sure if that's a good idea, though...
Or do you want to change the page background, i.e. the background behind your wiki page on click, i.e. within the browser window. Well, that's possible but it would require quite a bit of JavaScript development to get it working. Also not sure if that is really what your users want... so, yes, definitely possible, but neither easy not really user-friendly.
Or do you just want to open the image in a new tab, rather than in the media manager? If so then you can change them to "direct link" when inserting them...
OR do you want to change the page background for the site, in general? This can be relatively easily achieved by just a bit of CSS (see devel:CSS), I recon a line like body {background: black url(<path to picture>) center no-repeat; } or similar would do.
You can even set different image for light mode and for dark mode. See CSS media query prefers-color-scheme for more information.
But the way you have written your question, it could be any of these...
