It's super-simple when you know how:

Every time you change to a new screen, you open a different html page.

Each one of the subgalleries is a separate html document with its own filename.

Each individual image page is a separate html document with its own filename.

Each html document contains the html code for the images it displays and the links to the other pages.

So, instead of having one huge program to do all of this, you have a lot of little webpage files, one for each page.
To make it easy, make the first gallery page. Then "save-as" it to another filename, and change the image and hypertext links, and you have the second gallery page. Repeat this process to make all of the gallery pages. Use a similar process to make all of the image pages.