Here's a little trick I learned. Type the anime, videogame, show, etc's name or characters or whatever describes your music next to "wallpaper" in the google search bar. Then a bunch of wallpapers will show up in the search results and you can download one of them (yes DOWNLOAD them, not copy and paste the thumbnail images of) directly from one of the websites you find there. Hopefully you found the right dimensions for the wallpapers as well straight off the websites, cause that's required for ranking as well. And ofc, find something free, cause if you want to make a lot of beatmaps, you don't want to be wasting lots of money, right?