[Jelmertjee]

It should be simple, first of all you need a way to identify the categories, if they are forums you should have a look at some of the vbulletin code and search for a variable storing the forumid for example. You can then use that variable for template conditionals like this:
<if condition="$forumid == 1">current category image<else />standard category image</if>

The hard part might be finding that variable, I'm not sure how you intend to do the categories, if you're using forums look at forumdisplay.php or something like that. You can always show the content of variables on a certain page using var_dump($var), add a conditional to display it to admins only and in that way you can easily find the variable you need and test it without anyone noticing it except you.