Just use Google images for your search:
http://www.google.co.uk/imghp
When you see lots of possible pictures, don't bother with clicking through to the source page. Simply right-click on the picture (from the 'search results' page) that you like. What you do next depends upon your browser.
If you're using Internet Explorer, select 'Properties'. Triple-click on the address shown in the panel (to highlight it). Right click and select 'Copy'
If you're using Firefox, simply select 'Copy link location'.
Then go to Gravatar, right-click in the 'URL' field, and select 'Paste'.
Chris