Not an alt tag, a *title* tag. You'd add it to that image there:
Code:
<img src="$stylevar[imgdir_statusicon]/thread$thread[statusicon].gif" id="thread_statusicon_$thread[realthreadid]" title="whatever you want it to say" alt="<if condition="$show['threadcount']"><phrase 1="$thread[dot_count]" 2="$thread[dot_lastpost]">$vbphrase[have_x_posts_in_thread_last_y]</phrase></if>" border="" />