When creating or editing a Style Guide you have the option to add (Sample) Imagery for which you can choose between 16 different layout options.
User menu > Settings > Style Guides > Edit/New Style Guide > Style Setup > Imagery
The Imagery tab is setup by default when you create a new Style Guide. In order to change the layout you can click on the plus in the lower right corner and choose your desired layout option.
If you would like to add an Imagery section click on + Add followed by Add Imagery.
Please refer to this article for the specific dimensions for each layout option.