I'm not sure if this is a bug in Magento 18.104.22.168. I just tested with the default theme, but there's no difference, so I assume it's not theme related.
If I go to a sub-category page via main navigation, e.g.
http://mydomain.ch/maincat/subcat.html, the category image shows fine, but if I go to the same category via layered navigation Magento adds the sub-category as a GET param, i.e.
http://mydomain.ch/maincat.html?cat=123 and the category image is not showing. The products are showing fine on both URLs, though.
Any ideas if this is a bug or a feature? And if it's a feature the question would be: How to rewrite the latter URL to the first in order to show the category image on both.