I guess that you've been mislead by naming here.
REST is mostly thought of as a way to interact with resources over HTTP, by using HTTP verbs (GET, PUT, POST, DELETE).
The WMS-standard is described as RESTful (but if it's actually RESTful is another question).
Thus, your question can be interpreted as "is it possible to get a map image from Geoserver using REST?" In that case the answer is a loud YES: Using the WMS standard as described by R.K.
But, then, as our friends in ESRI decided to create their own "standard", the "ArcGIS REST API", which I think of as a "kind-of-wms" and then the confusion sets in.
Because, as you've apparently found out, GeoServer also has a REST api (in addition to the "WMS api"). But this Geoserver-specific API operates on Geoserver itself (not the Web Map Service it offers). So, using the "Geoserver REST API" it's not possible to get a map image, but that was never the intention eiter, as Geoserver serves map images using WMS.
Hope this clears things up!