This is something i learned on my journey to becoming a better digital artist. I normally do all my work using paint shop pro 8. I discovered gimp through a photography class i took when i did online schooling. This is a simple photo manipulation that anyone can do. Even the most inexperienced can do this. I found this tutorial on www.youtube.com.
First things first, open your gimp program. This is a tutorial for GIMP 2.6, im pretty sure it can work with past GIMP programs.
Now that you have the program opened. Go to your files where you keep you pictures and open the picture of the eye.
Over in the toolbox menu select you Doge/burn tool. It looks like a magnifying glass only its black. In the options menu select burn. Make sure that mid tones is selected. Then click on brush dynamics and make sure that everything is deselected.
Now with everything ready to go. Set your brush scale to the right size and start going around the eye until it has darkened to you liking. That you have done this go around the iris of the eye and darken it up. Do this until it looks completely black.
You may need to change your exposure scale high to spend less time darkening the eye. Don't worry about making it perfect.
When you are done with this. Click over to your layers window and create a new layer. Make sure you have set it transparent. Then click okay.
Now that you have you new layer. Lets change the color of the eye. Go to your toolbox window and select the paint tool. Pic a color you wish to change the eye to. I decided to go with red. Click back over to the layers window and set the blend mode to color.
Before you start coloring your eye, click back to the tools window and make sure you have everything deselected and your opacity set to 100.
Now begin coloring the iris.
Now that your eye is color. Lets add some cool effects.
Open your picture folder and find a cool background to use. I decided to go with a dried up creak bed. This is going to give the picture the effect of cracks.
On your picture that you just opened. Right click on it, go to edit, then copy. You can now close this window.
Click back to the original picture, right click, and select paste.
Okay, now that you have pasted you picture on to your eye. Click over to the layers window and where it says floating vector, right click and then click new layer.
Now that you have done this, click back to the blend mode drop down menu and select multiply. You should now be left with this. Looks cool so far huh? Well we are not done yet.
So now that you have a cool effect to you eye. Its time to crop the image. Click over to the tools window and select the crop tool. It looks like a little exacto knife. Now go back to your picture click and hold, drag you coursour all the way to the other end of the highlighted box. Then let go of you left click. Drag the sides in and out till you are satisfied with you selected area.
Make sure your image is crops inside the doted yellow lines.
When you have your selected area, dubble click on you left mouse button.
Now, in your tools window you are going to select the eraser tool. Set the scale all the way to 10.00 for easy erasing.
Click back to you eye and erase everything that is inside you eye.
TA-DA! You have no just edited a plain eye.
There are several ways that you can do this. Just apply what you know and have fun with it.
Remember, if you still have more questions, please feel free to ask.