Select this URL by clicking and dragging over it or clicking three times quickly:
http://lh6.ggpht.com/_aLQbInRJsGU/TS76AWFWooI/AAAAAAAAAMU/APA4FJ7_sd4/s128/IMG_0008.jpg
Now copy the selection to your clipboard:
(command c, or use the edit menu at the top of your screen to select copy)
Then go to the options page in your profile. Select the "I have my own pic" field by clicking it. (Make sure it's empty by clicking and dragging or clicking three times quickly and then deleting.)
Paste the contents of your clipboard into that field:
(command v, or use the edit menu to paste)
If the url is in that field click "change profile"
OR do it the hard way...
Open another window and just type it in by hand.
Control click on MACs brings up a Windows type menu. You can buy a 4 button programable mouse even for older MACs. I actually programed copy and paste for two of the buttons because I use 'em so much.