Changing collapse.gif & collapse_collapsed.gif

[effone]

It should point to your theme's directory by default. I guess you are using some theme with hardcoded path, which is not using the var {$theme['imgdir']}

Generally you can find it under template forumbit_depth1_cat:

<div class="expcolimage"><img src="{$theme['imgdir']}/{$expcolimage}"

You may have to change the path to some more places like UCP, Boardstats, Quick Reply etc.