The error is a bit misleading. It's not that you can't add raster imagery, it's that you can't access it directly as an individual layer from a map service.
There are two types of services that most people typically use: map services and feature services. Map services return an image of the data that is drawn on the server and returned to the client as an image. Thus, map services support complex cartographic symbols. Feature services return feature geometry to clients and expect the clients to draw them. In your particular case, by accessing the individual layer of a map service, the service returns the features of the layer, in the same way a feature service does. Since a raster layer has no feature geometry, it won't draw as features.
You need to access the map service as a whole, with all the layers, in order to see the raster data.
Thanks,
Mike
