Fortunately someone pointed out to me that they weren't seeing the picture I included in my last post, or I might never have realized that, in the process of all my uploaded pictures being transferred automatically from the now-defunct Picassa, everything that was once shareable became inaccessible to anybody but me, as does anything new that I upload, unless I specifically put it into a shared album. Which makes sense from a privacy point of view, I guess, but, man, that was not a graceful transition. And figuring out what had happened and what I needed to do about it was just confusing, since the description made "shared albums" sound specifically like albums other people could upload to, not just ones whose pictures you share with others. Turns out, when you make one, it asks you if you want to allow that uploading by others as an option. Would have been nice if that were clearer upfront.
Anyway. I think everything is working now. If you have any problems seeing images here, though, including in older posts, let me know.
(P.S.: Anybody know how to correct Google's mistaken identification of lots of non-car things as cars?)