See this:http://www.fsdeveloper.com/wiki/index.p ... x_gamepack
Also Google "GMax" for more info and sites.
I know very little about this, so wait and see what anyone else here has to say and what advice they have to offer.
I believe GMax came with FS2002, maybe others. It's a very complex program which is used for flight simulation aircraft design like what we see in FS9, FSX, etc. If you download it, you will see what I mean.
Other programs I used in conjuction with repaints were DXTBitmap:http://www.mwgfx.co.uk/programs/dxtbmp.htm
And 'Image Tool' is also helpful. Google it to find it. Both are free downloads.
For file format conversion, I used DXTBitmap and/or Image Tool. I believe Photoshop also can be used for conversion and repaints.
My own experience with repaints years ago (for myself, not worth uploading) came after doing photoediting and some other computer graphics things, so that prior experience was helpful, but despite my moderate computer knowledge, GMax was far beyond my reach.
My advice is to first learn how to repaint to get an idea of how these graphics programs work.
There may be online help for stuff like this.
It took me a while to figure it out. I don't do anything like this anymore, no time, and have forgotten a lot of details, but it was a worthwhile experience.