You should be able to ungroup map image layers.

MDB_GIS

I don't understand why map image layers are locked together in a group. I have map image layers from our portal to ArcGIS Online so that the public can see, but not edit some of our data. When I add that layer to my map, it gets added as a group and it cannot be ungrouped. This is very frustrating. 

There is a wonky workaround. If you add each individual rest service as an item to AGOL, then you will be able to pull the layers in one at a time, but obviously this requires a lot more setup and seems a bit pointless to have to go through all of that. 

Lwkrynock

I would like to see this implemented as well! We publish lots of map image layer web services. I don't understand why the ability to ungroup them isn't available, but can be done with feature layers. Here's a post I wrote with some more info on the issue, the workarounds we use and why they aren't ideal.