This has actually been covered several times. Either modify the css, or remove it from the FORUMHOME template:
HTML Code:
<a id="sidebar_button_link" href="#">
<vb:if condition="$show['sidebarposition'] == 'left'">
<img id="sidebar_button" src="{vb:stylevar imgdir_misc}/tab-collapsed-left.png" alt="" />
<vb:else />
<img id="sidebar_button" src="{vb:stylevar imgdir_misc}/tab-collapsed.png" alt="" />
</vb:if>
</a>