Just a note, I was having problems with my custom background showing up. I did everything correctly, but the background would always be black.
I was editing my image in GIMP and making sure to save as a 24 bit image, just like the source. After almost pulling my hair out getting this to work, I decided to try another image editor, just in case.
After using Paint.NET to edit the image, it worked perfectly.
So, I don't know if there was an additional setting I should have checked in GIMP or what, but thought I would share in case any other poor souls encounter the same thing I did.