Unlike Joomla's mind-f*ck approach in category setups, where the menu item exclusively controls the output of a category, in K2, when you link to a single category, the layout options come directly from the category settings.

If you have uploaded images with your K2 items (in the K2 image tab), by default you'll see these images in any category listing (or tag/user/date etc. listing). So unless you've disabled the option to show the K2 item image, it should be there.

However, if you're migrating from Joomla content articles and you have images embedded within your content (within introtext/fulltext), you need this little cool plugin to extract the first such image and "make" it a K2 image (aka serve it in a separate variable in the K2 item object): github.com/getk2/k2-iaki

If you like K2, don't forget to rate it at the Joomla Extensions Directory: extensions.joomla.org/extension/k2/