It is happening in davebreen.com/scenics and /haircut (so far).
I see the problem you describe in your 'scenics.html' web page.
The problem is caused by the custom CSS on your web page (which seems a little confusing) rather than any gallery configuration options.
Your gallery has a width and height of 100% so it should fill its parent container.
However, the gallery's parent container (<div id="Gcode21" class="dfltc">) does not have any dimensions set via CSS so Juicebox will be unable to determine what the gallery's actual size should be (i.e. 100% of what).
To add to this, the <div id="Gcode21" class="dfltc"> container is nested inside the <div id="Oobj25"> container which has been assigned a width of 50.00em and a height of 30.80em and the <div id="Oobj25"> container is further nested inside the <div id="centered"> container which has been assigned different dimensions (a width of 432px and a height of 100%).
Here are a few tips which might help.
(1) Try to simplify your web page's layout (use only as many containers as you actually need). At the moment, your gallery is nested within 3 separate containers, each with different (or no) dimensions.
(2) If using a percentage height for your gallery, then make sure that all parent containers of the gallery have heights set via CSS (up to and including the body tag). Please also see the note regarding Using Percentage Heights.
(3) Try not to mix em, px and % for container heights. Stick to px and % to see if this helps.
(4) You also have some custom CSS (below) which will be applied to all <img> tags on your web page, including those in your gallery. The gallery will have no option but to inherit such generalized CSS code. Apply your custom CSS rules to only those elements on your web page that require them using CSS id and class selectors (rather than element type selectors).
With all that in mind, I think a quick fix might be to just give your gallery a fixed height of, say, 600px (in the gallery's embedding code), e.g.:
Incidentally, when I try to view your /scenics/ gallery (on its own 'index.html' page), I currently get an error 403 (forbidden) error which suggests that the 'index.html' file might be in place but with permissions that are too restrictive. Default file permissions of 644 should be fine. You should be able to check and change file and folder permissions via your web hosting account's online file manager or FTP program.
Should the "index page name" always be "index/html" (since I'm embedding galleries using baseURL), or should it include the page name I've given the page of my site? I don't find this covered anywhere.
You can name the gallery's HTML page anything you like. As you are using the baseUrl method of embedding, all gallery files will be contained within the gallery folder so there is no chance that files will clash with conflicting filenames so you can safely leave the name for each gallery's HTML page 'index.html' (although you can change this for each gallery if you want). If you plan to embed your galleries in existing web pages and do not want to display the galleries on pages of their own, then the gallery HTML pages are actually redundant and can be deleted from the gallery folders if you like.