How come new comments don't bump the page up the tracker of recent changes but changing the image for example does.
Can you give an example? Where have you commented/seen comments that haven't bumped the tracker?
Just a thought, if you're not logged in, the tracker updates more slowly. So the comment wouldn't appear to bump it up the tracker until the cache updates. (This wouldn't apply if you made the comment though, unless you logged out immediately afterwards... which might be possible if you have strict cookie settings or something).