The best way of doing this is to find the image on the web and then right click on it then left click on properties and hilight the AddressURL) E.G.
http://www.cartooncars.co.uk/Cars/car%20images/115.gif

then right click on the hilighted address and copy it.

then you need to login to the passionford forum at the top click on profile and at the bottom of the profile page there is a Avatar section paste the address into the URL field and click submit.

Make sure the image is the right size as it states in the Avatar section do known whether it matters or not.

Hope this works.