You cannot link directly to an image stored on your machine. You must find somewhere on the Internet to host it first.
ImageShack® is a website that will let you do just that.
Go to ImageShack via the link above and hit the button labeled "Browse...". Find the image you want to upload and double click it. The location of
the image should appear in the text box to the left of the browse button. Then, you hit the button labeled "host it!". You will be taken to a screen
full of different lines of text: "Hotlink for forums (1)", "Hotlink for forums (2)" etc. Copy the text from the box marked "Direct link to
image", and paste it into the text box on
this page labeled "Avatar
URL:". Hit the button marked "Submit" at the bottom of that page.
You should now have a working avatar.
[edit on 12/10/06 by Implosion]