I have two Zenphoto sites with only very minor differences in their themes, yet when I upload images to one of the sites, the "visible" checkbox on the Edit Album > Image admin page is unchecked. Can I edit a theme file to ensure this is checked by default? Also, it would be nice if the "expand all fields" was checked by default. Thanks for any help!

Most people would not agree with you with respect to defaulting all fields to expanded. That presents quite a bit of (normally) useless information.

As to the visible, go to the "publish content" utility. There is where the defaults for this are set.