well if you are using gimp you most likely saved them as an xcf file. i think that is standard for gimp. if not and you saved it as a bitmap or bmp, then what you need to make sure is that you flatten the image to where you only have one layer. and make sure that the layer is in the indexed or 256 color. but i highly recommend the bright application. keeps alot of the detail in the skin and converts it to the proper bitmap file.
hope this is clear enough for you. if not im sure one the guys in the forums will explain it to you better than what i can. good luck with the skinning.
You know what the call a quarter pounder with cheese in France?
You don't need to flatten the image. I never flatten my templates as one mistake when saving will result in a ruined template (You did make a backup before working on it, correct?)
Do a File...Save As. then be sure to put ".bmp" at the end of the file name. GIMP chooses the file type according to the extension you put ni the name. If you're using a newer version of GIMP, you'll get another dialog where you can choose what kind of .bmp to save as. Choose 24 bit color depth.
You should end up with a 3MB .bmp file. If it's bigger, then you saved it with 32-bit color. Run the 3Mb file through Bright to reduce the colors to 8 bit (256 colors) and you'll end up with a 1MB .bmp that can be used in the game.
Bright is a command line application. Start up the Command Prompt, navigate to the folder with Bright in it, and type in Bright to run the application... if that fails type... I think its "Bright /?" (without the quotes) to get the help information.
Its command line plus switches. Its old school...but it gets the job done!