Robohelp recognizes and displays .png files; however, in RoboHelp 2015 > Tools > Graphics Locator, .png is deemed "Not a valid graphics file." You can search for .png in the Search For field above - and it will locate all the files - but it will not display a preview of the file in the Graphic Preview box. Can anyone comment as to why?
I'd nearly forgotten about that little application.
My own guess is that this application is a bit of a relic from days long past. Likely hasn't been updated in many versions and has simply been included in the install.
You might report it at the link below:
I wouldn't be holding my breath though.