vBulletin LTS

Download the two images and upload them into your forum/images/misc/ directory, using your FTP.

In the navbar template of yor forum style search:
(AdminCp -> Styles & Templates -> Manager -> Edit Templates)

Code:

 
  <!-- / nav buttons bar -->

Below, add:

Code:

  <!-- Register Button -->
<if condition="$show['registerbutton']">
<if condition="THIS_SCRIPT != register">
<script type="text/javascript">
function mouseOver()
{
document.b1.src ="images/misc/register_on.png"
}
function mouseOut()
{
document.b1.src ="images/misc/register_off.png"
}
</script>
<a href="register.php$session[sessionurl_q]" rel="nofollow"><img style="display:block; margin:0 auto; text-align:center;" border="0" alt="Join $vboptions[bbtitle]!" src="images/misc/register_off.png" name="b1" width="283" height="89" onmouseover="mouseOver()" onmouseout="mouseOut()" /></a>
</if>
</if>
<!-- / Register Button -->