Ah, the power of the web. I posted the problem I was having both here and on the WordPress.org web site, and within a week, another developer pointed out the simple one word addition to the plugin code to make it work in WordPress 3.1. Problem solved!
Now that I’ve spent some time poking at the code, and inspired by the “How to Build a Plugin” presentation at last Seacoast WordPress Developer’s meeting, I’m thinking of a fancier plugin that will let you load up as many images as you want and shuffle them. Inspiration is one thing, perspiration is another; I’ve got enough on my plate right now that this is a pretty low-priority issue. And there’s the challenge of feature creep: it would be nice if I could link each header to page that described the significance of the picture, had its licenses, etc. And that you could load up multiple pictures and enable/disable them individually for seasonal themes or special events. And… oh, dear.