Have you ever used Paint Shop Pro? I use it exclusively mostly because it is very afordable and with some clever techniques you can make some great art with it. Unfortunetly, it is no where near as sofisticated as Illustrator and Photoshop.
As for .PNG files, I never could make .png files work right with SFC (I use .bmp's). On the other hand, I hardly play SFC anymore anyways so...