Well, you can use any picture from any site, so long as it's <20k in size, and < 110x120 pixels in size.
If you're in a website and see a little picture you'd like to use, then just right click it (assuming you're using Internet Explorer) and select properties.
This will show you the size and dimensions, and also the path to the picture,
http://www.site.com/images/pic.jpg or summat like that. Copy that, and then use that as the link to your avatar in your user profile.
Also, there's some sites out there that have loads of avatars, eg
freeavatars.net.
If you get really stuck, just email us the pic you want to admin@ukbouldering.com, and we can sort it out for you....