A: 

well I ended up using the iframe option of ThickBox to load my image with another page. I don't love this solution (it requires a separate page and extra code to determine the optimal height/width of the iFrame)... but it works and I don't have any more time to mess with it

Nick Franceschina
A: 

http://drupal.org/node/140371:

In the Thickbox module, the main *.js file, thickbox.js does not account for image urls created in drupal, e.g. via /image/view/2321/preview instead of /image.jpg (or *gif or *png). If thickbox.js does not see the file name itself (gif, jpg, png, jpeg) in the link anchor tag (the [a href ...] tag), it will treat the image like another mime type that is not an image. This causes Thickbox to spit out God-awful garbage on top of your page instead of a cool shadowbox (lightbox) effect.

Looks like the same issue you're having.

Brother Erryn
wow... good job... this was right on. I modified those two lines in thickbox to add ".ashx" extension, and it works! since this is the REAL answer to the question, I will accept it... but I'm not actually going to implement this solution (I'm going to stick with the iFrame) because I don't want to deal with a custom thickbox script
Nick Franceschina