http://www.gimptalk.com/forum/pop-out-signature-tutorial-t9171.html
Today, we are going to learn how to make your render "pop-out" of the signature area. It gives a really cool effect and is really simple to do. When we are done, it will look a little like this.
OK, to begin, you need to prepare your render. I am going to use this one.
Feel free to use it if you want to.
Open up a new image with the dimensions of 400 x 125. Now you will need to make a background. I'm not going to give any real steps on how to do this, but if you look around on this site, there are many tutorials showing you how to make them. Try using this one, as it is the way I am going to make mine. Don't add the render to your background though. We will do that later.
Following those steps you will get something like this. If you want to add a border, now is the time to do it. The link to the tutorial I told you about above will also show you how to do this.
You now have both a background and a render. Press Ctrl + A to select everything. Now go to:
Image ---> Canvas Size
A window like the screenshot below will appear. If you made the size of your image 400 x 125 like I told you, simply set the settings to the same as shown. If you have used a different size, double the width and height of your image and type what the numbers come to into the boxes names "Width" and "Height". Set the offset X and Y values the same as in the screenshot.
Press "Resize".
Your canvas will now look something a little like this.
This is just how you want it.
Now for your render. Open it up and it will appear in a new window. If its to big, resize it. Now go back to your signature window and make a new layer. Copy your render and paste it on to your new layer. A handy tip to know is when you press F11, it will bring The GIMP into full screen mode allowing you to see what you are doing clearly. You should get something like this. Yours will have little dotted squares all behind it though like the image above. Its OK, its how it should be.
Now add some text or whatever you want on your signature. A cool way to make text look good is to type what you want out on to your signature in white, move it to where you want it to be, set that layer's mode to "Overlay" and then duplicate it untill it looks nice. Heres what I got. Again, squares should still be present.
Now merge all the visible layers by pressing Ctrl + M. Select the first option. Now right click on the layer in the layers window and select "Alpha to Selection". This will select all your signature. Copy it by pressing Ctrl + C. Then go to:
Edit ---> Paste as New
This will open up a new window with your signature in it. Go to:
File ---> Save as . . .
Save it as "Popout Signature.png" it must be in a .png for it to work.
Thats it your all done! This is what I got.