Alright, sorry for the triple post.
I have the solution to my problems, and I managed to figure it out, despite the lack of assistance.
If anyone using RPGmaker 2000 has the "Invalid size" error, check the folders you're trying to import to. Chances are, that's 90% of your problem right there. If you're having the "Unsupported" error, save to .bmp extension. Not .png, not .xyz, it has to be .bmp.
Anyway, onwards to the completion of my game! (hopefully...)