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^
I was wondering how you upload your texture with Gimp 2.8. Please help.
File > export. Then save it as .jpg or .png and upload what you saved then. The typicall .xcf which is a gimp only file cant be uploaded anywhere.
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