like the smaller picture in song select?
make the first difficulty you made's background the image you want small, make a second difficulty and give it a different background, then upload the mapset while in the editor for the first difficulty (unsure if you have to be on that difficulty in the editor or if it just takes the oldest difficulty's background).
the second diff will have the first's background in the smaller song select picture, but its actual background will be the background you gave it. the first diff you made will only be able to have one BG.