Bug/problem for you. If you open threads in a new tab in firefox (not checked other browsers of consequence), using the middle mouse button or "open in new tab", the unseen tab does not resize the images. When you switch to the tab in question you see the image at full size. The only way to have it resize the images is to reload the tab that you're looking at.
I'm using the TCattd mode with highslide - I haven't tried other settings.
For an example, open
this link in a new tab but don't give it focus. If you give it focus at all while it's loading you won't see the problem.