RMXP is a bit different from the others (i.e. 95, 2000 and 2003), it uses the Ruby Scripting language, so you might want to look up that stuff. Sprites can be in *.gif format or *.png format I think, I haven't exactly looked into this RPG Maker recently - last I heard was RPG Advocate translated it and is trying to crack the software protection.

Anyways, hope that helps a little.