Stay away from BMPs--that's the biggest storage hog.
Your problem with saving is either the color mode or layers. Chech that you are in full RGB, not indexed color. Also make sure your flatten the image to save it as a BMP, GIF, or JPG. It took me a little while to figure that out & was highly frustrating until I did.
'95 Black PGT -- work in progress (many mods, many to come)