Thanks, Mr. N! I followed what
this post recommended. I wasn't sure what "From the launcher" meant, but found Manage Applications from the menu when at the Home screen and it looks like I did what it said and now Aldiko doesn't disappear after using the USB cable.
I still need to know where exactly I should copy image files. Back in another lifetime, when I was still a DOS newbie, I would just copy stuff into the root directory of my C: drive, but then learned to make directories and keep stuff organized. It just goes against the grain to dump pictures just anywhere and depend on Gallery to find them.