I looked at a LOT of forum threads before posting this:

I like the fact that images can be viewed at full size by clicking on them. However, I do not see any method to return to the normal image display page or the album.

When I click on an image to display it in large format, the only way back is with the 'back' triangle at the top of my browser tool bar.

Can someone tell me how return navigation is usually achieved?

BTW: I'm using the Basic theme.

The full image is generally outside the theme pages and displayed directly. There is a "protected view" that is displayed via the core full-image.php but it is still not directly theme context.

Full images might be generally too large to be directly displayed so the sized images on the theme image.php page should be very well be sufficent for viewing. You can however modify the image.php page to use the full image.

Thanks for filling me in. May I ask if it is possible to turn off the full image response to clicking, since some viewers may feel they have become 'stranded'?

....Or, if there were a way to assign a watermark only to the full-size view, to consist of directions to use the browser's 'back' button.

You could simply activate the _colorboxjs plugin and check image.php in its options for the basic theme.
A watermark is also possible. Look for options and directions in Admin => Options => Images => Watermarks
And you could of course also decide to modify the theme.
https://www.zenphoto.org/news/theming-tutorial/

Yeah, too lat yesterday night so I forgot to mention the obvious solution of using the colorbox plugin for an overlay.

Red face: somehow I did not see the tiny 'x' page at the bottom/right of full-size images that allows visitors to return to the normal z-p image.

I will look into modding the theme; I want the main div to be wider.