Hi David,
Of course we are aware of this limitation. The lack of transparency effects for workspace child windows is due to a Qt bug that remains unfixed for years, and I have nearly no hope to see it solved. So unfortunately, translucent image windows are only supported by PixInsight for Windows and Linux/UNIX, but not by the Mac OS X version.
That said, I am always trying to improve PixInsight's GUI on all supported platforms, so this limitation may eventually be overcome. Sorry for the inconvenience.