Make sure you have view avatars turned on, click on user cp (top of the page, on the right, underneath the home bar and just underneath the forum breakdown bit) scroll down until you see the edit options link on the left, and click on that.
Scroll down a fair bit until you get to the thread display options box, inside that is another box called "Visible Post Elements" with three tick boxes, one of which is for avatars, make sure thats ticked and save your changes.
That should display the avatars, if not try emptying your cache and checking your browsers security settings to make sure it isnt set to block images from non trusted sites. If that still hasn't fixed it or you dont know how to change security settings and empty your cache then phone a friend
or you can PM with your browser and i'll try and walk you through it. (to find your browser details click help at the very top then click the bit that says About ***something*** )