Notes

In this short tutorial, I'll show you how to tweak underexposed photos easily using the GIMP for Windows and a few of its plugins from GIMP plug-in registry. Should be usable for Photoshop users as well.

Step 1:

Rotate the picture if neccesary, get rid of the frame or edges you don't want (if there is one) using the crop tool.

Step 2:

Now we can start with the real thing:
Select the layer with the original, then right click -> Add alpha channel

Step 3:

Duplicate the layer 2 or 3 times and make the original layer invisible. I've made two copies of the original one.

Step 4:

Set the top layer to screen mode and copy it as many times as neccesary (you don't want to overdo it, the aim here is to make things that were hidden in shadows visible). After this step, you should finally see some things happening to your picture, it should be brighter. I've used 3 layers in screen mode in this case.

When you're done with it, merge the visible layers.

Step 5:

Make the original layer visible again and move it to the top.

Step 6:

Your picture should look like your original underexposed shot now. Now right click it and go to Script-Fu->Photokit->Dynamic Range Extender and set the.

Step 7:

The photo may look washed out now. We'd like to enhance colours, contrast, etc., but first you might want to apply unsharp mask (Radius: 0,5; Amount: 1,00, applied twice in this case) from the filters menu and selective gaussian blur to both sharpen the shot and get rid of the image noise.

Enhancing colours is easy using the Vivid saturation filter from Photokit in Script-Fu menu.

Step 8:

The photo should look much better now. The sky could use some stronger shade of blue, though. Manipulating colours directly probably won't be what we need, since it often produces unwanted artifacts. Therefore, we'll get back to layer manipulation.

You should have only one layer now. Duplicate it twice and make it invisible, just like when we were starting. This time, however, we'll use Overlay Mode on the top layer, I've duplicated it once after that. Merge visible layers.

Step 9:

Using the lasso tool, select as most of the sky. You don't need to be very precise here, just roughly copy the tree line. When it's selected, right click your image and select Selection->Feather... and set it to 20 pixels. Then press ctrl+I to invert the selection.

Step 10:

Press ctrl+L to make the selection a new layer, right click it and go to Layer-> Delete layer to delete it.
Make the bottom layer visible again and we're done :D.

Final comparison:
Final image:

Original image: