This instructable is aimed at a range of people, so you can skip to the bits relevant to you with the information below. I apologise for the verbosity of the main instructions, but I figure you can skip parts you already understand, and I may as well put in the detail for people who don't.
If you don't know what High Dynamic Range (HDR) is or how it works, read on.
If you are comfortable with the concept of HDR and want to know how to do it in photo editing software, go to step 3
If you are familiar with the process of making HDR images and just want details of how to do it in the GIMP, go to step 4
If you are familiar with the interface of the GIMP and just want a quick set of instructions on how to make HDR (for instance if the theory bores you and you just want to make some HDR images), go to the recap stage in step 10. The details of each step are in... the relevant step.
If you know the theory, how to make HDR images and are familiar with using the GIMP.. I'm not sure why you are reading this, but hello anyway. My pitiful attempts (I'm relatively new to HDR and the GIMP) are in the later steps, perhaps you could give me some pointers? :D
Lastly, if you find this instructable helpful (or even not particularly) and have constructive suggestions for how I could improve it, let me know and I'll edit it.
Step 1: Theory- photographic
A camera, like the human eye, has a light sensor that captures the image. The sensor has a limited sensitivity to light, so under any given conditions there is a range of light levels that the camera can differentiate between. It doesn't matter if you are looking at the sun or two suns, your camera will throw up its little electronic hands and say "that is white". Similarly, unless you have a very flash camera or night vision, looking a dark grey bat in some slightly darker grey surroundings at night time, your camera will say "that is black". The difference in light level between the darkest levels your camera can distinguish between and the lightest it can distinguish is called the dynamic range- for a regular digital camera it is about 100 to 1, for the human eye it is in the range of millions to one.
The "dynamic" part is because the range can be altered, by opening or closing the camera iris (aperture), opening the shutter for a longer or shorter period (exposure), and how much the camera amplifies the signal on the light sensor (ISO). The ability to change the camera's range of light sensitivity is what we will exploit to create HDR images.
The dynamic range of a camera is not great, and not a patch on human vision. Things that you would see as dark, the camera will tend to see as a black silhouette. Things that you see as bright, the camera will tend to "wash out" and see as white. This means that taking photos of subjects with a high range of light levels, for instance a nice sunset, is very difficult with a regular camera and a single exposure. In making HDR photos, we exploit the ability to move the camera's dynamic range over the entire range of light levels.
By taking a shorter exposure or closing the aperture, the camera will underexpose. This means that the dark parts of the scene will silhouette and the light parts will be recorded well. Taking a longer exposure or opening the aperture, the camera will overexpose, meaning the light parts of the scene will wash out to white, but the shadows of the scene (that were black in the underexposed photo) will come out well. Composite HDR refers to a range of techniques to take the best parts of both of these photos and combine them in a photo that captures more of the range of light levels in the scene.