Custom Shadow Map Crashes Game

I'm making a CC accessory jacket in the necklace category and want to make a shadow map for it but every time I test it in-game it crashes. I've tried everything and followed every tutorial I could find but it just doesn't work. I use GIMP so I can't modify the alpha channel like it says to do in some tutorials but a lot of people have been able to use GIMP anyways so???

IDK if this is a bug in S4S Star since it's in beta but if it's not, please help

Thanks 💕


The Shadow Map in question (screenshot with reduced quality)