Alt text has a real semantic meaning, so I didn't want to do that
Thanks for following up!
My intention was to only show the default Alt-text if no custom Alt-text was set. Kinda like how the throbber is set up now.
I was hoping that basically no one was using images with transparency right where the throbber would go, but I guess not.
My Throbber test in the OP also shows a few pixels through the transparency.