With the stock Gallery app, how can I open an arbitrary image file by specifying its filesystem path?
You're not limited to using the Gallery app to view your images.  I've loaded a file explorer app called ES File Explorer File Manager that allows me to traverse not only the local content of my phone, but, also, any network drives I may have access to.  When selecting an image file, you are presented with a list of programs that are registered to open the image.  The program can use the Gallery app, or, its own built in image viewer, or, any other registered image viewer.