I just installed the new Diigo on my Firefox. And i realize every time there is a picture on any web page, once put the cursor on it, a "save" button will come out by Diigo for saving the current picture. This is cool, and I really want to know how Diigo can detect pics and save them.

http://help.diigo.com/premium-features/save-image-from-a-webpage
this web page explains the function I described above better...
I am trying to look through the source codes of Diigo 5 to get the answer.
bt if someone who knows much about Diigo can give me tips, then that will be more helpful!!