Repainting aircraft: What Software to use?

I enjoy FS2004 very much but would like to repaint some of the aircraft.
I have experimented with the texture files but all that has happened is I screw up the default paint. When I select the aircraft, it shows up as basically a gray silhouette. How do I repaint? Do I need a photo program of some sort. Keep it simple because I am not a puter whiz but I am tenacious and will keep at this til I get it right!!! Thanx

By the way I have been a pilot for 32 years and FS2004 is very enjoyable for an 'Old' jet jockey...