Here is how I configure my Alt+Print Screen action for taking window screenshots:

First, install gconf-editor if you don’t have it already:
$ sudo aptitude install gconf-editor

Then, open gconf-editor with this terminal command:
$ gconf-editor

Navigate to:
/apps/metacity/keybinding_commands/command_screenshot

Change the ‘command_window_screenshot’ value to:
gnome-screenshot --interactive --window --delay=1 --include-border --border-effect=shadow

Now when you use Alt+Print Screen to capture a screenshot of a window, you will get a nifty little drop shadow behind your image by default. See the example below.