I have enabled the three images sizes and Use full screen expand, but it always show the menus at the top and the bottom.
All Showkase pages (under the Boma, Kosel and Maribo themes) always show the navigation menu.
This is by design (so that visitors can easily navigate throughout the site).
The only way to have a full-page gallery within Showkase is to use the Gallery Manager theme.
However, the Gallery Manager theme supports only Juicebox and ListViewer Gallery pages (not About, Basic or Contact pages) and does not display the navigation menu on any pages.
If you really want to have a full-page gallery within your website, you could perhaps set up two separate Showkase sites (one using the Kosel theme and the other using the Gallery Manager theme). Next, create a Juicebox gallery page in the Gallery Manager site and then use a Navigation Link page type within the Kosel site to link directly to the Juicebox gallery page in the Gallery Manager site. You could include a Back Button in your Juicebox gallery to allow visitors to seamlessly return to a chosen page within your Kosel site.
Please note that the imageScaleMode configuration option (in the Main Image section) in the Multi-Size demo gallery is set to FILL so that the images always fill the gallery's image area no matter the size or shape of the user's browser window. (Cropping will occur if the aspect ratio of the image does not match that of the browser viewport.)
I also noticed that I cannot seem to cascade Gallery index pages.
Showkase supports only two levels in its menu system. Gallery Index pages are always top level pages and the Gallery pages listed on a Gallery Index page are always second level pages.
Unfortunately, it is not possible to nest Gallery Index pages under other Gallery Index pages.