thickbox

I am trying to use thickbox code with my gallery2 site. However when I click my thumbnail I get all garbled output in the popup window. I got the thickbox code working ok but its not displaying images correctly. I am thinking its cause the image links dont end in gif or jpg? I got it working as iframe content but then the images cant be resized every time. Any help would be appreciated.

this is my rendered code:
<a href="main.php?g2_view=core.DownloadItem&amp;g2_itemId=17" class="thickbox" rel="images">
<img src="main.php?g2_view=core.DownloadItem&amp;g2_itemId=16&amp;g2_serialNumber=2" width="150" height="150" alt="DSCF0018"/>
</a>

this is the output i get in my popup window:
�����JFIF�

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 2!!22222222222222222222222222222222222222222222222222���
��
"�

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}
�!1AQa"q2���#B��R��$3br� 

Jim