I finally found a copy of the original icon (I thought I'd lost it on my home computer, but apparently it survived several migrations on my work computer and was safely hidden away), but even though I can clearly see the picture in my files, when I added it to my userpics all over again with a different comment, LJ says I've successfully added it but still shows no picture.

Oh, well . . . ! At least I know it's not lost forever!