Could you check whether the same happen on default Joomla! template like Beez ? The settings for category/image description are inherited from menu item, category and global settings so maybe just it is not set properly in all this places.
And in category configuration all is fine ? If the issue exist on Beez it is not related with the template but with you configuration. Do you have menu item related with this category view ? Please check also the settings there because probably this ones are the most important.