URL = Uniform Resource Locator.. Or in other words, the web address of a resource -- e.g http://www.somesite.com/image.jpg (probably not a real example)
And, in order to have the image show up in your profile you have to either upload the picture to a web-server which you can remotely link to, or find one on the net*, then put the address in the URL field.
* Some images found on sites such hosted by companies such as Geocites or Tripod cannot be linked to remotely, and other web server administrators may not like you remotely linking to images hosted on their servers, as it consumes some of their bandwidth.
