Creating a master page is a simple task, and should consider as soon as possible in the layout process. The number of master page you'll need will depend on your choice. Let's see how to add your new master page and have them applied.
Click on the first left-hand side button of the Edit Master Pages window.
You'll be prompted to give a name and reference side (we choose Right Page) to your master page: give a simple but explicit name like GalleryRight (try to avoid spaces and special characters like any é, ü, or so).
A new empty master page is listed in the window and the page appears in Scribus in place of the page you were on previously. It will have the default document margins and the default size.
Customize it with new objects. Eventually change the page properties if needed.
Draw a small rectangle in the top-right corner (see the next illustration if you need an example). If you wish, you can go to a ModelRight, copy a rectangle, and paste it here...