I encountered this issue today (Psychopy 3.1.5) and I solved it by changing how I saved the png.
Apparently Psychopy can’t load an image if the color profile is saved into the file. There may also be other things that Psychopy stumbles on. Saving the png with GIMP’s default export settings should solve any issue with using a png as an image.
Definitely make sure to NOT select “save color profile” (highlighted with the cursor)