I highly recommend Adobe Photoshop Pro. It is like the Gmax of paint programs. It has so many 'effects' things you can do to layers, filters also, which are basically effects, each one tuneable for angle of light, shadow, etc.
With Photoshop (PS) you can then go to the nVidia website, developers center, and download the tools that work with PS, such as the DDS editing addon that allows you to work on DDS textures and save them, including their Alpha channel. Another tool is the Bump Map compiler. This will compile a BM (no pun intended) or 'bumpie' from a gray bumpmap draft you made, then switch the Alphas around to work with FSX.
You can get older Editions of PS, such as 6.0 on Disc format now and then from people selling their CD's at Ebay.com and Amazon.com, but they are getting rare. CS4 of PS is really nice but pricey. If you can afford it its worth it, but thats if you can afford it.
But.. DXTBmp will convert them also. I like speed. I need speed. Being able to do 'all' DDS work in one location is awesome and saves me by using one less compiler. However, I work in FS9 too, so I use DXTBmp daily for the FS9 graphics.
Bill