Do you know about the power on .png? You are saving your work as a .jpg, which compresses the image, lowering the quality. If you save your images as .png, you retain the quality of the original image that you made in GIMP. The only reason you might save an image as a .jpg if if the file is too large, which only really happens with avatars, but there's a way to lower the KB, making them usable on ZD.