It is a known issue that Safari can have problems displaying images when they are png. As a work-around you can try using a different browser:
Firefox- https://www.mozilla.org/en-US/firefox/mac/
Chrome- https://support.google.com/chrome/a/answer/7550274?hl=en
You can also try the suggested fixes found online:
https://smallbusiness.chron.com/clear-mac-caches-websites-visited-51822.html
https://appletoolbox.com/safari-images-not-showing/