Something that every unix should have on it already is xwd & xwud. You could make a script to perform this function for you, then bind the script to a function key. In gnome this script might look like:
#!/bin/bash
set ans=`Xdialog --title "Dikk-let" \
--inputbox "Push a screen shot to who?" 10 50`
xwd -root | xwud -display ${ans}:0.0
The script prompts you to type an ip address or host name to send a picture of your root display to someone. If you enter nothing, the screen displays on your machine.
This assumes you have Xdialog installed, i.e. sudo apt-get xdialog
I named my script "dikk" and saved it in a directory where I put little scripts like this -> ~/bin. Given that, you can add a custom from System -> Preferences -> Keyboard Shortcuts
