Yeah, I may have simplified too much here. MacDraw II (or maybe it was already ClarisDraw at the time?) had an option to make a bitmap transparent. I don't remember if it just used a pen mode that wouldn't draw white pixels, or if it actually generated a mask internally using some seed fill algorithm and set that as the clip region in the resulting PICT file when you exported it. But any way, there was a way to generate transparent bitmpap PICTs despite not having an alpha channel.That’s a nice overview. “PICT resource with a mask” wasn’t really a thing, though. A PICT resource was just a sequence of QuickDraw instructions that were followed to the letter. So if you pasted your PICT from an object-oriented “draw” program like MacDraw, and those instructions didn’t include whiting out the drawing area, you would get a transparent PICT. (Whereas if you pasted your image from MacPaint, you’d get a bitmap inside your PICT, which by definition whites out all the white pixels as part of rendering the bitmap.)
If you pasted a bitmap into MacDraw then copied that into a PICT resource, you would still get an opaque bitmap.
Of course QuickDraw made it possible to draw bitmaps with a mask in general, which one could have accessed in HyperCard via an XCMD. I’m sure someone must have created a way to do this at some point.