For questions regarding the image displayed on the desktop background.
The tag excerpt of the second says:
Questions about using desktop backgrounds in Ubuntu.
So the tags are essentially about the same thing, i.e. about desktop wallpapers/backgrounds.
In the past it was proposed that the background tag should be burninated because it was also used for issues with background tasks/processes, which led to the creation of the background-process tag. However, the background tag is still in use for desktop backgrounds.