Right - no that's not in the head template then - it's in something below that, but I'm not familiar with that template I'm afraid. It could be skin1/home/main/welcome.tpl, but it looks to be above that too.
So it could be in skin1/customer/home.tpl
If you use Webmaster mode, you will find the template very quickly.
You need to search for the class itself class="MainImageBackground" - possibly on a <table> tag or a table cell <td> tag.