You can't upload textures with gimp, only make them and save them.
But what I'm guessing you're asking is how to upload textures made in gimp?
First off: You'll need the "Texture Items" required to use the specific texture, like if you want to upload a head texture you'd need a 128, 256 or 512 res "Head Texture Item".
When you're finished creating your texture in GIMP, you can save it on your computer as a .BMP .GIF .JPG .PNG .TGA or .TIF file.
Then do what Fenris said^
Helping admins since 2013 (Can't ya'll leave this alone?) Reta's Bitch, Property of Kriegery, Bribed by Bird, Tinerr 4 President.
Awaken, awaken, awaken, awaken. Take the land, that must be taken. Awaken, awaken, awaken, awaken. Devour worlds, smite forsaken