Well, the way I do it is: I mostly use Safari and have an iBook, so I just hit CTRL and click my mouse, choose "Open image in new window" from the drop down menu, then copy the address from that new window when it opens, and use the IMG button here and paste in the address..
When I use Firefox, I still hit CTRL and click my mouse, and it pops up a menu with a Properties option. I choose that, and copy the address from there, and do the same IMG/pasting here