How would I take a 24-bit png and bump it up to a 32-bit one?

[Crash]

I've been doing some retexturing with Rice's plugin in Project 64, and when I try to load a tile I made from scratch, it won't show up in the game. However, I've noticed that the textures (png in this case) have to have 32-bit color depth, and not 24. Textures I edit directly with a program (ie: GIMP 2) have 32-bit data and load fine in the game.

How would I bump the color depth up to 32-bit? If anyone knows, I'd appreciate it!

Edit: Hey I really need some help here; unless I'm asking a stupid question or something. This isn't hard to do, is it?