Hello Thilo,
You can use the K2 Tools module and the Categories List (Menu) Functionality.
This however does not display the category image.
For what you want to achieve you have two other solutions as well.
a) Use the Catalog (parent category) to display a list of all of it's subcategories with images and introtext.
You can choose not to display any items if you wish as well.
b) If you need it for a sidebar or a module elsewhere, follow the exact steps given on the post and use Joomla! menu item's picture and notes to display your category image and introtext.
JoomlaWorks Support Team
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Please search the forum before posting a new topic :)