That's what I thought. But I don't see any other reason for that. Maybe the forum uses a different "technique" to render the image in that page?
The way tracking pixels work is by linking a image which is hosted at your website, and you have direct access to the logs of the visitors of that image. By introducing a proxy there's a third party involved to prevent this. At least, that's how I'm assuming this is being dealt with. I know. I mean, what if in that page the image would just be loaded from the original website instead of passing through theymos' proxy?
The preview page? Well, the poster of the image would be the only one seeing it as its in the preview so at that stage there's no reason to proxy it. That's if I'm understanding the question correctly. 
