Is there an easy way to display a PNG image (with transparency) as its own “window”, without any OSX window features (title bar, buttons, shadows)?
Ideally, it could also disappear when the mouse is clicked anywhere, or when Esc is pressed…
Yes, this is possible. See Apple's RoundTransparentWindow code example:
This sample demonstrates how to create windows with custom shapes, no title bar, and transparent content. It also shows how to change the shape of the window and recalculate the drop shadow around the window border.