As of right now there is a possibility to set a Forum Image for Categories while creating/editing it. But there is no place where it to gets displayed. So there are two way of 'fixing it'. That setting could be removed from the form while creating/editing a Category which I'm not a fan of or it could be added on index(Pic. no.1) and viewforum pages. While on index it fairly simple by just using existing code. It can be done by altering this

<!-- IF forumrow.S_IS_CAT --><a href="{forumrow.U_VIEWFORUM}">{forumrow.FORUM_NAME}</a><!-- ELSE -->{L_FORUM}<!-- ENDIF -->

to

<!-- IF forumrow.S_IS_CAT --><!-- IF forumrow.FORUM_IMAGE --><span class="category-image">{forumrow.FORUM_IMAGE}</span><!-- ENDIF --><a href="{forumrow.U_VIEWFORUM}">{forumrow.FORUM_NAME}</a><!-- ELSE -->{L_FORUM}<!-- ENDIF -->

in forumlist_body.html file.

On the viewforum the template variable is not available so the image doesn't display.