I recently did a photo shoot for a wedding. I will show you how to do some enhancements to the photos using the GIMP.
Step 1 - Duplicate layer
We want to duplicate the background layer. We probably won't need to use this, but it is good to have a backup and a reference point.
The image is quite flat right now, so we want to bring out some of the levels. Go to Colors>Levels
You can do auto level at this point and that usually works quite well, but I want to get it a bit more exact.
Choose the black eye dropper and pick the darkest region on her hair.
Now pick the white eyedropper and pick the whitest region on the white flowers.
While we have the levels dialog up, we will lighten it a bit and increase the contrast.
Step 3 - Vibrancy and Saturation
Let's make the colors a bit more vibrant.
Open Colors>Hue-Saturation... Increase saturation (I did 20).
Step 4 - Fine Tuning
You may be happy with your image now and can skip this step. If you want to do some more fine tuning, you can go back to levels and edit some of the levels. I usually edit some of the color channels (like red) to get the color more how I want it. You can also do more adjusting with hue and saturation.
Step 5 - Soft Glow
Duplicate your layer.
On your new layer, add a Gaussian blur. Filters>Blur>Gaussian Blur...
Blur with a value of 4 or 5.
Now change the layer mode to "Screen" and turn the opacity down to about 25%
|Layer mode to screen|
Step 6 - Add a vignette
Create a new transparent layer.
Select the gradient tool. Choose FG to transparent.
Draw the gradient over the image. Start where you want your hightlight to be (in this case, the bride's cheek area) and draw to just outside the screen. Turn the opacity down to about 30%
Here is the before and after: