Using v1.3.9g on an IIS7 server.

I am having trouble with the pop up larger images. They are displaying at the setting for my small images i.e when you click on the link to them, they open up as eg 400px × 550px (scaled to 100px × 90px)
Here is a page that shows this
http://tooltraderdirect.co.uk/index....roducts_id=195

I am using the ImageHandler mod (v2_0-Rev8c), but was having this problem before installing it, thats one of the reasons I added it...so I believe it is a more general issue with my Zen install rather than the mod. I am also using the Silverfish template.

hope someone can help