I found a typo in the Openbox howto. Let me quote the relevant paragraph: PCManFM is the lightweight filemanager from LXDE. It supports tabbed browsing, drag and drop, thumnails for images, bookmarks, volume management, searching, and more. It also provides supports for managing the desktop background and drawing desktop icons (both optionally). Note the "thumnails" instead of "thumbnails".
wow nasty bug :), pcmanfm has more problem then just this :)
Fixed, thanks for reporting.