TL;DR If you are getting crappy looking png images from rmarkdown html or word documents, try using type=‘cairo’ or dev=‘CairoPNG’ in your chunk options.
PNG Graphics?? So, I write a lot of reports using rmarkdown and knitr, and have been using knitr for quite a while. My job involves doing analyses for collaborators and communicating results. Most of the time, I will generate a pdf report, and I get beautiful graphics, thanks to the eps graphics device.