I use GIMP too and have the same issue. There is an easier way....
Create a seperate folder to store all your graphics. Name that folder after the name of the blind (will explain why below) Put it somewhere that you can get to easily. When you save an image, save it a .png and put it in that folder.
Then it's just a matter of importing it into Skinstudio. When you do, it will automatically convert it to .tga. If you need to edit the image, just edit the .png in that folder. Then when you're done editing, all you have to do is click on the 'force image reimport' button in SKS. It remembers where you got the image from, and updates it.
The advantage to this is that when your blind is done, you have a folder full of all the bits you used to make it. You can store them somewhere, so if you want to make something that matches the blind, you have all the graphics you will need to do it!
Note: Naming the folder is important! If you name it something generic, like "bit's and pieces", everything will be fine on the first blind you make. But if you make another blind and use the same folder for storing those bits...it will be ok, UNLESS you open your previous blind in skinstudio. If you do, your previous blind will automatically import parts from the one you are currently working on, into your older blind.
So, always name you folder after the blind you are working on, so the name is unique.