Hi Tara - What I was able to do at 10.5.1 was to install ArcServer Advanced, and then authorize it for Server using the Advanced ECP. At the Advanced level, you can see that Image Service is a supported service type:
I then re-run the authorization for image server using the ECP for that role, which then basically enables the Image Service service type as a licensed role so I can publish mosaic datasets (among other tasks). I'm getting ready to do the same thing at 10.6, I don't think that this method has changed.