Below are links to tutorials using Gimp as your imaging source. I've used Gimp personally so i don't really know how it works but i hope these help.
Creating Clothes With Gimp
Make a Tattoo using Gimp
Fabric Folds in Gimp
Make a Tshirt in Gimp
Jeans Tutorial Using Gimp
If i find any more i'll post here.
Enjoy
Kagome
