I have an issue with the collapsing archives plugin when using "images" as expand and collapse characters in combination with the ~-sign in URLs. When the archive tree first loads the images appear correctly. However, when the javascript kicks in (both by clicking on the sign, but also with auto-expanding), the images disappear and are replaced by the words "expand" and "collapse". If I look at the image URLs I see that part of the path that contains the ~sign got removed.
So when the tree loads the collapse/expand image urls are correct, e.g.:
http://example.com/~wordpress/wp-content/plugins/collapsing-archives/img/collapse.gif
But after expanding (and collapsing), the image urls suddenly are replaced by
http://example.com/wp-content/plugins/collapsing-archives/img/expand.gif
So without the ~wordpress part and thus incorect URLs. I found this behaviour in both Firefox 3 and Internet Explorer 8, so I suspect it is a bug in your script.