Well, I sort of found out why this is happening. On my machine, I have maybe a half-dozen applications that can open images.
As often as not, merely opening an image with one of those will change the existing "default" opener for that image type. And
the new default will not necessarily be the program I just used, or the pre-existing default. For instance, a bit ago I had all my
.jpg files showing the PDN icon in Explorer, and I used the "Open with ..." menu to open one of them in Windows Paint. I closed out
Paint, and restarted Explorer. Now, all the .jpg files are shown as "IrfanView JPG File"s. Anyway, whenever an image is associated
with PDN in Explorer, and I choose the Print action, it seems the command registered at HKCR\ paint.net.1\shell\print\command is executed,
which opens both paint.net with the image showing, and the Print Pictures Wizard on top of that. I just deleted that "print" subkey,
and now everything works as it should.