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You mean a BG that doesn't have animations? If yes, for not having the anim. there are 2 options: Import a PNG on a slot that doesn't have anim. or 2) You have to sacrificate Minigames (or unused stuff/or add new files to the ROM). In few word with an old
version of NSMBe you can use every file as BG, for example a minigame stuff is ID 200, you select 200 as custom BG, it loads file 200, 201, and 202, which 200 is ncg, 201 is the ncl, and nsc is 202. I haven't tried it myself...