Hey!
So the gallery screen is a default renpy game menu, so this will depend a little on whether you've done customizations to the general menus
You can try this modification to the screen in step 8, but your background here will be underneath the default game menu background (configured as gui.game_menu_background) unless you've removed that background or otherwise changed game_menu already:
screen bobcgallery(): tag menu default pagenum = 0 add "album" use game_menu(_("Gallery"), scroll="viewport"):
The better way would probably be to modify game_menu itself.
1) Make this change to screen game_menu in screens.rpy:
screen game_menu(title, scroll=None, yinitial=0.0, bg=None): style_prefix "game_menu" if bg is not None: add bg elif main_menu: add gui.main_menu_background else: add gui.game_menu_background
2) Now update bobcgallery:
screen bobcgallery(): tag menu default pagenum = 0 use game_menu(_("Gallery"), scroll="viewport", bg="album"):