Which Google Drive Images Are Added to Google+ Photos?

If you're wondering why not all the image files from Google Drive are available in Google+ Photos, here's the explanation. Google+ Photos only imports photos if they meet these criteria:

1. photos MUST be at least 512 x 512
2. JPG photos MUST have 'Date Taken' EXIF metadata
3. supported formats: JPG, GIF, WebP, RAW.

I've tested various image files and these are the rules. Google wants to restrict the feature to photos, but it's not clear why GIF and WebP are allowed, while PNG isn't.

{ Thanks, Martin. }