There are some programs that allow for exporting STL CAD files used in 3D printing as simple PNG images - in so called slices. What this does, is basically slice the 3D model of the part and save each slice as a PNG image. Default function in some CAD files, but you can also probably find some 3rd party utilities or scripts designed exactly for this job.
Convert Stereolithography CAD file to Portable Network Graphic.
We have found 3 software records in our database eligible for .stl to .png file format conversion.
Microsoft Windows software - convert stl to png on Windows
A 3D modeling tool
Convert from stl file format
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned stl to png. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
A widely-used 3D animation application from Autodesk
A 3D CAD translation softwate