I am working with PowerPoint 2016 for Mac

PowerPoint can export your presentation as a series of images. Go to file, export, select PNG, and you can select just one slide, or the entire presentation to be exported. In the latter scenario, images will be saved in a newly created directory.

Probably a left over from earlier PowerPoint versions, the resolution of these images has always been poor when using the standard settings. In previous versions of PowerPoint, you could somehow change DPI (dots per inch), but it did not affect the output. There are also ways to hack registry system variables (on a Windows machine, not a mac). The results have always been unpredictable.

In the most recent version of PowerPoint on mac, you are presentation with a menu in which you can enter the desired slide dimensions. This dialog behaves strangely when entering extreme values has height or width, flipping the orientation of the slides.

For some reason, I get decent pictures both in 4:3 and wide screen aspect ratios when setting the width to 2998, and the height is calculated to 1686. I have tried to understand why, but failed to do so far. It is probably not worth breaking your head over it, just use these numbers.