Regular png does not have spatial reference, and therefore adding it as an operational layer in a map does not render the layer. It can be added as a base map layer.
There is an exception though. If the png file is created from ArcGIS desktop and comes with an .aux.xml metadata file, the spatial reference and extent information can be stored in an xml, in which case, the info can be read.
For the more common case when it's just a png file, there is no spatial reference.