The gallery displays all images for the currently displayed item. For example, When in the main menu, the gallery will display is all images; when viewing a Field Entry, selecting the Gallery will show all images for the field entry and its children.
The folders tab in the gallery shows all items below the current level.
Long-pressing on an item in the gallery will display a menu showing the owner of the image and its ancestry. Each menu item will display the relevant item details.
The default icon sizes can be set via Settings.