WildWEEGEE wrote:
9. Save as "npc-101" or whatever you want your sprite to replace in .gif format
10. Colour your sprite completely black and make the background white
11. Save as "npc-101m" or whatever you want your sprite to replace in .gif format
This is only the case if you're working in SMBX 1.3
If you're working in SMBX2, you should save the file with transparency in the .png format, and do not create a mask file.