There are several good image editors out there, such as
Paint Shop Pro X or
Photoshop, both of which are very intricate and take some time to learn (and cost a buttload of money as well!

).
You could also try
FS Repaint, which was designed for repainting in FS specifically. It will show you the aircraft as it would appear in FS, as you paint it. In 3D and everything! It doesn't cost too much, and it looks pretty cool . . . I've never tried it, though. Free trial lets you repaint stuff but not save it. It also cuts out the need to know how DXTBmp (or any other converter) works.