A URL (Uniform Resource Locator) is a web address. (For example, the URL of the page you're viewing now is
http://www.theanswerb...g/Question951672.html ).
The following assumes that you're using Internet Explorer:
Go to a web page where there's a picture you want to use as your avatar. Right-click and select 'Properties'. There you'll see the address (URL) of the image (rather than of the whole page). Triple-click on that URL to highlight it. Right-click on the highlighted address and select 'Copy'.
Now go to the Gravatar page where the 'URL' box is. Right-click in the box and select 'Paste'.
Chris