Let's say, you just finished making a model and you want to make an icon with it.
We need to make it render with a solid background. I prefer black. It doesn't matter what color, as long as it's a solid color.
I'll be using black for this tutorial. " I just don't like that shade of blue."
In the button box, click on (1) Shading,
and then click on (2) world.
Now, click on the color swatch (3) and choose a background.
a color picker dialog pops up, choose black.
You should be looking at, something like this:
Click on the scene button, and let's go set some render settings.
Blender isn't good at cleaning up after itself. It has a tendency of leaving files behind.
By default the temporary directory, is where renders are saved to. Here, I create a new folder, and save to it.
Then after I'm done, I delete the folder.
This is where we setup the render picture. The first thing we need to do is: setup the render width (1).
Then we need the set the render height (2).
In the drop-down menu, set to PNG (3). If you are a Windows user, you can change this in the gimp later.
If you noticed, I set mine to 120 x 120 pixels. I got some strange results, when I set it to 80 x 80 pixels.
But we will get it in the ballpark, later.
I find that most desktop icons, range approximately 80 x 80 pixels.
Adjust your lighting, and filled the area of your camera with the object. But don't go beyond the Tile Safe.
Hint, set your viewport draw type to (shaded).
With this draw type selected, you will be able to see how lighting effects your object.
and check your results.
Once you are satisfied with the results. Save your image.
File/Save Image
I saved mine to the desktop, and gave it a name.
When you have your image and .blend file saved, close Blender and open the Gimp.
Close the tip of the day.
I like to minimize my layer window to the clipboard, I like to keep it handy. But thats a matter of preference.
In The Gimp window, under File/Open, navigate to your image.
Now that you got your image open. Go to The Gimp window, and select the(Magic Wond.)
With the Magic Wond Tool, select the black in your image.
Then go to the Select menu and choose, Invert.
Hard to tell the difference, but what it did was: selected the object, instead of the background.
Now, go to the Edit menu and select Copy. Then once again, go to the Edit menu and select: Paste as New.
And 
As you can see, the object was pasted with a transparent background.
Also, you can see on the title bar, that the pixel size has changed. Now, were in the ballpark.
It seems to me it is cropping the background to the object, or some kind of resizing.
In any case, if you are not satisfied with the size of your icon. Go back into Blender
and move your object either, closer to the viewfinder or farther back in the background.
Did you save your .blend file?
When you're satisfied: Save As, to the directory of your choosing.
Linux
And 
A dialogue box will pop up, asking you to confirm.
Except the defaults, and click OK
Windows
And 
This is surely not the only way to create icons. This method just gave me the results I wanted.
I hope this helped, have fun!
