It looks like a K2 template override issue. Your template probably bundled K2 template overrides and perhaps something is outdated there. You can always compare what your template has for K2 with /components/com_k2/templates/.

The overrides in your Joomla template may also include the "default" template, which would explain why you don't see things change much.

You need to either compare the files there with K2's source template files or find the folder html/com_k2 inside your template and rename it to html/com_k2_backup. This way you'll know you're using K2's original templates.

Just rename com_k2 to com_k2_backup inside your template's /html folder so you can at least see how your site looks with the default set of K2 template overrides.