The filenames for the custom levels are “byo_level_fg.png” “byo_level_bg.png” make them the same size as one another and place them in the RebopBlasters_Data directory (should be in the same place as the executable).
The two images need to be the same dimensions, but can be quite large. “byo_level_fg.png” will be loaded such that anything with above zero opacity will be solid, destructible terrain. “byo_level_bg.png” will be placed behind and is a great place to put dumb memes.