Gimped! Gimp Tutorial: Photo Tweaking 101: How to do basic photo tweaking using GIMP

In this lesson, I'm going to show you the basics on how to edit a photo using GIMP.  We're going for realism here, so the changes are going to be very acute in contrast to a lot of my other posts, although a lot of acute changes add up to a big difference in the end.
Part 2 can be found here.
Suggested Prerequisites:
Layer Mask Gimp Video Tutorial
Layer Mode Gimp Video Tutorial
Adjusting Colors Using Gimp
Layers and Selections Gimp Video Tutorial
Suggested Items:
Wacom Tablet (For Shading and Masking)
My eBook on Layer Masks - This short and inexpensive book focuses on one thing -getting you insanely good at making Layer Masks.  If you get good at layer masking, you'll get good at Gimp.
Here is the photo we're working with:

Click on this photo to download the original version.

All in all, the photo is a good shot.  I personally don't think there's too many things wrong with it.  The girl's eyes don't have much pop, the guy's eyes are semi-bloodshot, and the background seems to be a little on the busy side.  With the use of layer masks, layer modes, and tweaking opacity, we are going to try to make this couple pop just a little better.
Part 1: How to use GIMP to Fix a Smile
First off, we're going to whiten those teeth.  They're not too bad now, but you'll be surprised at how much better they can be.
Create a new Layer, set the layer mode to "color." Name the layer "teeth."
Create a layer mask on this layer. (Right-click on the layer, click "Add Layer Mask") Set it's color to black.
Change your color palette to black and white by clicking on the black and white squares in the bottom-left corner of your toolbox.
Click on the arrows near the color boxes to switch your foreground, and background colors.  (Shortcut Key: X) You should now have a white foreground color, and a black background color.
Using the paintbrush tool (shourtcut key: P) and a soft brush, slowly "paint" the color out of the teeth.  Be sure you're painting the layer mask, and not the layer itself.  If you make any mistakes, switch over to the color black, by pressing the X key.  Painting black in a layer mask will restore the color.  When you want to remove color again, simply press the X key again.

The layer mask black-white relationship allows for quick fixing, tweaking, and fine tuning.  It's a few extra steps to set up, but I recommend getting in the habit of using one whenever possible.

After you're finished with one set of teeth, go ahead and do the same to the other set.

No matter how clean a mouth is, the teeth can always be a little bit cleaner.  Desaturating the teeth is an effective way to clean them.

 Duplicate the teeth layer.  Set the duplicated teeth layer mode to overlay.  This will brighten the teeth up quite a bit (probably too much, in fact.)

Adjust the opacity to something that looks realistic.  I ended up setting my opacity to 40.  Here's what I ended up with:

The teeth have been whitened.  Who needs toothpaste, anyway?

 Part 2:  How To use GIMP to Enhance Eyes

With a lack of proper equipment, the both of their irises seem to disappear.  The man's eyes are a little bloodshot too.  Using almost the same methods as we did with the teeth, we will help bring their eyes out in this photo.

Create a new layer, name it ManEyes.  Fill the layer with White.

Set ManEyes to the layer mode overlay

Add a layer mask to ManEyes.  Fill it with black.

Make sure you're editing the ManEyes layer mask, and are brushing in white.  Fill the iris (not the pupil) with white.

Although it doesn't always work, Overlay will often pull natural color out of the eyes.  Sometimes you will need to tone down the opacity, although in this photo this is not the case.

Create a new layer.  Name it EyeWhite.  Fill the layer with white.

Set the layer to Saturation

Add a layer mask to EyeWhite.  Fill it with black.

Make sure you're editing the EyeWhite layer mask, and are brushing in white.  Fill the white portions of the eye with white.

Removing color is often as powerful as adding color.

Adjust the opacity to something that looks realistic.  I set mine to 60.  Here is what I ended up with.

The change is slight, but a bunch of slight changes add up.

Now for the girl's eyes, which seem to almost disappear altogether.

Create a new layer, fill it with white, name it GirlEyes, and set the layer mode to overlay.

Create a layer mask for GirlEyes, and fill the mask with black.

Make sure you are editing the GirlEyes layer mask, and are brushing in white.  Using a large brush, Fill the girl's sockets with white.

Be careful with how much of the shadow you pull out with the overlay.  If the skin looks orange, you probably overdid it.  Press the X key, and start brushing the orange skintones away.

Adjust the opacity on the layer until it looks realistic.  I adjusted mine to 50, and ended up with this:

Another small difference, but they add up.

Create a new layer, fill it with white, name it GirlIris, and set the layer mode to overlay.

Create a layer mask for GirlIris, and fill the mask with black.

Make sure you are editing the GirlIris layer mask, and are brushing in white.  Fill the girl's iris with white.  The photo should end up like this:

Now that we have the eyes lightened up to an amount that we can work with, it's time to really pull the color out.

Duplicate the GirlsIris layer, and adjust the opacity to something that makes the photo look realistic.  I set my opacity to 30, and the photo looks like this:

The duplicated layer gives you control over a little extra boost of color in her eyes.

Select the background layer

click colors>>>hue saturation

Increase the saturation by an amount that works for you.  I increased it by 25.  Here is what I came up with:

Adding saturation to an outdoor photo will really bring some pop to the photo.

Part 3: How to use GIMP to Make the Couple Pop

In the final portion of this tutorial, we will show you how to make the couple pop out of the background without looking fake.  The trick is to pull color away from the background, not add color to the couple.

Create a new layer, fill it with white, and name it saturation.

Add a layer mask to the saturation layer you just created, fill it with white.  This should remove all color from the photo.

Using the lasso tool (shortcut key F),make a selection similar to the photo below.

The goal of this selection is to get the majority of the couple.  We'll use another method to get the edges.

Make sure that you're using the bucket fill tool (Shift+B), working on the layer mask you just created, and are using the color black.  Fill the selection.  Your photo should now look something like this:

Zoom in, and using a soft brush with the paintbrush tool, start painting color into the edge of the couple that the fill did not get.

You could also have used the pen tool, but in my opinion, a soft brush works much nicer in this case.

After your couple is completely in color, and the background is completely black and white, adjust the opacity until the background looks realistic.  I ended up with an opacity of 50, as shown below:

And that's it!  The differences aren't huge, but not all photo-editing work is meant to make, or break a photo.  Often times, it is only there to improve on slight issues with a photo.

OK photographers, what would you have done with this photo to make it better?  How would you have edited this photo?