Query All Media in a Category

Hello,
i couldn’t exactly read from the source code how to get a list of all media from a category from php.
To clarify, i am building my own theme and want, to have all the background images in their own category, so now i need to query for all images of that category when rendering index.php.
So what is the preferred/supported way to query by category then?
For now i am using WP_Query to fetch all images that have been uploaded.

Thanks in advance!