There is a default image for those who have not set an avatar:

However, the problem you are referring to is for certain members who had avatars in the old software that have somehow been lost, but the link remains, for example none (http://originaltrilogy.com/forum/avatars/custom/avatar-857.gif) or Trooperman (http://originaltrilogy.com/forum/avatars/custom/avatar-1516.gif).

In Firefox, the missing avatar is shown as a short wide box that breaks the forum table. In IE, you get a white box with a little red cross; the box is sized the same as the other avatars and keeps the table layout intact.

The easiest fix would be to get all the members who have missing avatars to either delete them or upload a new image.