Hey guys,
Looks like RawTherapee (the official web site appimage, as well as the latest appimage I downloaded from nightly, RawTherapee-dev-5.5-131-g04f8fc6-20190221) will not display PNG images in folders. They just don’t appear. 
I used to be able to edit png files if I remember right.
I recall some of my PNGs crashing RT and / or dt: perhaps one of the reasons that png is disabled.
@heckflosse THANK YOU! I never looked too deeply into the settings page and so I missed this list. 
@afre if it crashes i’ll do what I did earlier today; saved it as a TIFF then processed it