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Hi Warren,
You're absolutely correct in that the dark frames will be taken with the lens cap on.
So, if your exposure is 30 seconds long, make your dark frames 30 seconds long, too. Try to take them immediately after taking your normal (light) exposure as you want the temperature to match correctly.
I honestly don't know if The GIMP can do it. I know in Photoshop there is an Image > Calculations menu in which you can perform image arithmetic.
Might be worth a Google!
Good on you for figuring out blurring (depth-of-field). In general, the smaller the aperture (the larger the f/-ratio), the more depth-of-field (less blur), and, the larger the aperture (the smaller the f/-ratio), the less depth-of-field (more blur).
You'll note the image details in my landscape images -- I often shoot f/11 to f/16 for greater depth-of-field without softening my images due to diffraction softening. For portraits, you'd use anywhere from f/1.2 (whatever your lens can handle) to f/8 (or greater, if you've got a lot of people staggered throughout a compositional field).
Have fun!
H
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