IMCE buttons problem depending on themes

Hi guys,
I have a problem with the IMCE filebrowser depending on the theme that I use. If I use the garland theme everything is working correctly (see: http://i28.tinypic.com/ny9pfk.jpg) but if I use my own theme the buttons disappears (see: http://i28.tinypic.com/autau8.jpg).
What's the problem? How can I solve it? Any idea?

Thank you for your help.

Nicolo'