I was looking into SMBX and SMBX_Redrawn config packs and noticed some strange things. In SMBX config pack, in a folder "background", there are copies of background-1 and background-1m, but in png format, while every other file is gif. Next, I don't know why, but there's this effect-54 and effect-54m in the same folder for some reason. This effect exists in the same folder, in SMBX_Redrawn config pack too!
I recommend you to check every other config packs, if they don't have same or similar problems.