I know this is basic101 type stuff, but there may be some that don't know how to do it.
I am currently on the KOA rom from the .79 ICS leak.
It has the Apex launcher.
Not sure if it will work on other roms or launchers like this.
Here are the steps to make it full screen.
1. Take screen shot
2. Load the screenshot folder in the gallery and scroll over to it.
3. Press the menu button.
4. Select Crop
5. Drag the blue box to the top left corner and drag it down so it covers the whole picture.
6. In the upper right hand corner select crop.( Sometimes it may say loading for a long time, if it says it for more than a minute go ahead and hit the back arrow to go back to gallery)
7. Now you have your full screen picture.