Simplified theme - why is header image disappearing?

Hello,

I have embedded gallery2 (using the simplified theme) in my joomla website and everything seems to be working well except...for some reason, when I click on one of the thumbnail images in an album, my website header image disappears.

It looks like an extra index.php/ is being inserted into the URL bar. IE. before I click on the thumbnail, the URL bar says:
http://genkigaijin.com/index.php?option=com_gallery2&Itemid=31&g2_itemId=24
(and I can see my header image), but after I click on the thumbnail, the URL bar says:
http://genkigaijin.com/index.php/index.php?option=com_gallery2&Itemid=31&g2_itemId=26
(and I can no longer see my header image).

If I then edit the URL and remove the additional index.php/, the header image returns.

Does anybody know how I can stop that extra "index.php/" from being inserted?

Thanks.