OK, I downloaded 1590 for Mac.
I made a new png, Opened my panel in 1590, deleted the old resource, unlinked the image in the unButton. I then reimported the png resource and relinked the image to the button.
I saved the panel as a restricted Instance, on opening the Instance the image was still not there.
Atom or Msepsis can you recommend exactly what png / jpg specification is know to work best with Ctrlr. Maybe I am not saving the png correctly.