Hi,
by default there is not possible. You have to customize code of K2.
Probably it can be made by checking procedure (guide for your php developer)
User add new K2 item
and add image ---> save & publish (OK)
FALSE
---> he didn't add image ---> choose & use default image from ABC source for K2 item
p.s.
I have found this topic:
http://getk2.org/community/New-to-K2-As ... item-imagethis forum is only about K2
use them