HDR images are normally shot with your camera set up on a tripod and several images are taken of the same view in various lighting conditions. Then you layer the images on top of each other and blend them. If you want to learn how to do proper HDR images in Gimp try Flickr – Discussing HDR in Gimp. You’ll find everything you need there. Or try the Gimp blending exposures tutorial.
But if you are like me and taking wildlife shots you will find that the wildlife generally isn’t too keen on sitting and posing for several hours while the sun moves. Or maybe the local security officer is wiggy about you being parked in front of a building for several hours with a camera. So you only have one image to make it work.
First make a copy of your photo and open it in Gimp. ( You know better than to work on your originals right? )
Open up a layers dialog ( Dialogs->Layers )
Create a copy of the layer by clicking on the double image button at the bottom of the layers window.
Hide the top layer by clicking on the eye.
Select the bottom layer by clicking on it.
Go to your image and select Colors->Levels then Auto->OK
This makes your whites whiter and your blacks blacker. Don’t worry about whether it looks ok or not.
Go back to the Layers window and click the eye to make the top layer visible again
Click on the top layer to select it.
Now adjust the opacity until you are happy with the image.
On darker images you might find Mode->Overlay works better
Also you might also try adjusting just one color in your color levels. In the Levels box Channel has a drop down menu – you can select all or a color to adjust.
On some images I adjust the levels on the top layer instead of the bottom. You want to experiment a bit and see what works best with your photos.
Another option is to open Filters->Lighting effects and add a light to one layer before blending the photos.