Thanks, I've turned on the viewer_size_image plugin . I've set three widths: 475px, 800px and 1024px (full size), as $w=475; $w=795; $w=1024; .
With the Zenpage theme, I don't see an option to change the image size.
With the Flickrish theme, I see four options (including the thumbnail), but the size that I expected to be 795px wide is 1000px wide.
With the default theme, the image is coming up full size (1024px), with the options to change size smaller underneath. When I select a radio button, the image collapses into a horizontal line. When I refresh the page, the image gets loaded full size again.
The basics of Zenphoto work well. The extensions and themes seem to almost work, so it would seem that a better immediate tactic is to stick with the basics.