Manipulating Your Images in GIMP
Start GIMP and open the middle range exposure image. This is your base image. Open up the dark image in a different window. Next, go to Layer and then New Layer. Give your layer a name so that you know that it is the darkest layer. The middle range image must be the background layer. If it is not, change it to the background image.
Copy the dark layer. Select your dark image, and go to Layer, Colors and then Desaturate. This will make the white sections of the image opaque. You can lighten the layer a bit if it is too dark. Add a Layer Mask to the dark layer. Paste the mask into the image.
Next, open up the lightest image in a new layer. Desaturate the layer, but then invert it by going to Layer, Colors and Invert. Copy the light layer and create a layer mask. Paste the mask image.
Merge the layers to create the HDR image. Experiment with the contrast and gradient to make the image even more dramatic.
This GIMP HDR tutorial only gives you a brief overview of how you can create dramatic HDR images. If you want even more interesting images, take images at multiple exposure levels. This creates deeper colors and contrast. Also, experiment with the opacity and colors.