The specific way I make textures is:
Make stripes of varying hues and shades with grass brush.
Use motion blur a lot.
Ctrl-C current image. Drag it ~halfway out of the image (horizontal).
New layer. Ctrl-V. Drag that so it matches up with the previous image.
Ctrl-E (in Photoshop) to merge layers. Motion blur until texture is seamless.
Good luck! Gas giants are my favorite part of making systems.
Actually here is a link on a planet texture generater for photoshop. It's very easy to use and can generat very good textures.(level 8 is the the best) Lunarcell. It will make Bmp's witch you can then convert into tex files with the Pltex tool in your orbiter utilities directory. Have fun planet building!