How to write ExifTool keywords which Spotlight can see?
As of OS X 10.11, this appears not to be possible.

JPEG but not PNG

That you can not find a working example suggests the Spotlight indexer does not extract the meta-data from Portable Network Graphics, png, files.

According to Searching EXIF data with Mac OS X Spotlight, it does work for jpeg files.

File an Enhancement Bug

Report an enhancement bug through Apple's Bug Reporting web site – and through general OS X feedback. This is the best way to directly tell Apple what you want.