Here's the Firefox extension addon page:
https://addons.mozilla.org/en-US/firefox/addon/search-by-image-by-google/
The image.google.com function looks through the page and inserts it's own additional code whenever it finds an image. A 'camera' icon 
(which is my guess what the boxes are) on the found images, which when clicked auto sends you to google's image search. So when the 'insert/edit picture' function is invoked, the new code is found and the i.g.c extension does it's thing.