This tutorial Shows how to get Google Image search as Firefox search engine.Actually not only Google Image but with this tutorial add any search to Firefox in two steps. 1) Download this plug in and install and restart Firefox. https://addons.mozilla.org/en-US/firefox/addon/3682 2) Open any search page you want to add. Say http://images.google.com/. Right click on the search text box as shown on the screen shot and click "Add to search bar". Done!
This is how it looks after you add it 
Very easy and very handy specially for web developers and graphic designer who want to get staffs at easy convenience.
You can also choose what image you want to display as icon.
This is how it looks after you add it 