Thank you, JohnMart. Now I see just how it's supposed to work, and I followed your instructions to go down the file system and find Gwenview and select it, and
much to my amazement, the fix sticks in the "Open with" box. (I haven't rebooted, but other wise it sticks.)
I'm still convinced that the way this operates is faulty. How is the user to know what directory to open to find the program he needs? Yes, you can find it with locate, or
in the GUI with the Find Files/Folders app, but you shouldn't have to. If the "other" choice were a list of the programs in the PCMenu, as I suggested in my first post,
none of this would have occurred. With all the fiddling that Mozilla keeps doing with their programs, you'd think they could at least fix this!