In the add_button function, assignment of the image component is pointing to $link and should probably be assigned to $image.
I saw someone already post about the missing stylesheet, and went to download the files from your site. All went well. I did notice chu_r_b.gif and chu_r_t.gif were missing. Easily fixed by flipping the chu_l_b.gif and chu_l_t.gif files.
Besides a few minor css tweaks to meet some personal preferences, everything else looks good. One thing that might help is when navigating to a new category that a default button is pre-selected.