I find DXTBmp easy to use, like all MW's marvellous utilities. Of course, if you're new at this it obviously takes a while to figure out & get used to the different FS texture formats. Read DXTBmp Help for information on FS texture formats & compatability with the various sims. I usually save textures in their original format, whatever that might be. The way I look at it, if the original works so should my repaint.
The format will be shown at the top of the DXTBmp window when you first open the texture. For example: this scenery texture is in DXT1 Extended BMP format.

There are various tutorials on repainting aircraft for FS2002 & FS2004. This one will give you a good start.
http://www.calclassic.com/repaint.htm