The easiest way to get the URL for an image is to upload the image to a hosting site such as photobucket. Then you copy the URL that it gives you and paste it where it asks for the URL.

I'm sorry, Blackearth, but I have to announce that taking images from Google is illegal. Truly. You have to get stock images, which are images that you are allowed to use. I suggest going to Flickr.com, clicking Advanced Search next to the search bar, then checking the boxes that say Creative Commons License & 'Find content to adapt and modify upon'. You can also go to sobi.org for cat and dog images. deviantART also provides stock, so long as you search in that category. =P

Sorry to burst your bubble. But you CAN go to Photobucket.com and search for an avatar there. They have well-made ones that are allowed to be used.

A URL is the domain name of a wesite or image. If you would like to get an avatar, you could search through google images. Once you find a square-shaped image you like, click on it until you only see that image. You then just copy the URL and paste it in the bar that requires the URL. Or, you could save the image by right clicking it or going to "file>save as" and save it where you can search and find it when you select a file to send in as your avatar.