Technically, it is straightforward. It is just replacing the form background. I used to have this functionality and removed it because it makes it harder to understand what is happening when there are a lot of icons on the screen.
Whatever image was the default would have to stretch to fit the resolution of the user, so it would be blurry for some people unless cropped to the right size, which means it would be down to each user to find the best image for them, or for people on the forums to create them for specific resolutions. Also, finding 100+ images that all add background flavour without interfering with the display would be tricky.
Bear in mind it is literally a two minute task for me to add my own background and I don’t do it 
If you find a suitable image, I can show you what it would look like.