The Moodle mosaic on the front page of my installation of Moodle4Mac 2.7 didn't survive when I upgraded to 2.9. There was a broken image icon where it should have been, but when I tried to fix it (turned on editing, clicked the gear icon) I got a delightful error message from Moodle that was less than helpful.
Thanks to your post, I was able to determine that the folders that should have held the graphic were completely missing. I created the folders, grabbed a copy of the png from another install of Moodle, and all was fixed!
Thanks for your instructions. Even though they were for a completely different problem, they pointed me in the right direction.