Hi, is the Bf109 for IL2?
If so then you simply need to convert your final artwork to an 8 bit BMP format. This will reduce the file from 3MB to 1MB. Palette conversion for IL2 is often done using BRIGHT, since it's converter seems to outperform several graphics editors built in filters.
Here are some links from our tutorial library to get you started.
http://www.simmerspaintshop.com/foru...ng-bright-832/
http://www.simmerspaintshop.com/foru...e-artists-453/