I recently saw another thread that recommended deleting a line from /components/com_k2/templates/default/item.php ....
I had been trying to figure this out for a while myself! and I forgot that I found an answer in a different Joomla Docs Forum.
Here is what I did:
(In Joomla 2.5)
Extensions >> Language Manager >> Overrides
Click on NEW
In Language Constant enter
K2_MEDIA
(or K2_IMAGE_GALLERY)
LEAVE ALL OTHER FIELDS BLANK and hit enter (I assume you could add text if you wanted "Media" or "Gallery" to display something else, I wanted it to display nothing)
Attachment not found
(I also added JGLOBAL_ARTICLES so that the Joomla "Articles" header would not show up on other pages. )