Custom skins let you replace Mario's sprites. The template is included along with the game, so you can edit it to your liking and then save it as "skin.png" in the game's folder. The game will automatically detect and reskin Mario for you.
Read skinsTutorial.txt in the download for more information.
First screenshot: I fixed that bug in 1.8.1, now you can get "crushed" by the tanks if you stand in the left corner.
Second screenshot: I changed how the save game system works in 1.8 because the older system was broken. If you use a save from an older version it will display the save as "World 0" and crash. Make sure to delete mafosav.ini and restart the game.
[EDIT 1.8.2019]: I released 1.8.2 to fix some last minute issues, everything should be working fine now.
IMPORTANT NOTICE!!! There was a game breaking bug with the save pipes. If you cleared all your progress the pipes showed "World 0" and crashed if you entered them. I have fixed this issue and released Demo 1.9.2. Please re-download the game if you want to get rid of this issue.