// vBulletin 3.6.x

Try the Welcome Headers Mod.

OR

If you don't have a license, you can manually apply some code.
Go to AdminCP > Styles & Templates > (select skin) > Edit Templates > 'header'

Place this code where you want it to be:
<if condition="$post[usergroupid] == 1"><a href="register.php"><img src="file"></a></if>
Note: I've used UsergroupID1, as that's the default Unregistered usergroup.
Change "img src" with where your image is hosted.


// vBulletin 3.7.x

In the AdminCP, there should be a "Notices" tab, where you can add a notice for guests by default.



Hope that helped.

- UltimA