<p>I need to add some simple image renders of STL files to a document.  I currently open the STL files in Preview or one of the slicers and grab a screen shot.</p>

<p>Is there an easier or automatic way to generate PNG images from STL files on a Mac?</p>

Typically you would install a (free) 3D model program as Fusion 360, FreeCAD, or many more options to choose from. Once installed, import the STL file and use menu options to export a picture of your STL.
Alternatively, if you have some programming skills, you could
import the STL file
in OpenSCAD and render and export a picture from there. Simply create an OpenSCAD file with the code line below and it will import your
```
example.stl
```
.
```
```
import("example.stl", convexity=10);
```
```
Through the menu you can then export the view to an image. Note that you can do that also from the
command line
as shown by the OP's own answer (nice example of command line usage of OpenSCAD).
These are not the only options, there are many more. E.g.
this
is a nice example. It also describes how Thingiverse.com does STL to web image.