Hi Michael,
A Map Image Layer is the entire map service. For example, a map service may consist of many layers. When you add a Map Image Layer to a web map, you are not specifying an index value at the end of the URL, so all layers are added.
A Feature Layer is one layer within a map service. For example, you specify an index at the end of the URL.
Map Image Layer
http://ags/arcgis/rest/services/Parcels/MapServer
Feature Layer
http://ags/arcgis/rest/services/Parcels/MapServer/2