Navigate to Guidelines and click "Edit pages" on the left side of the screen. If you have not yet created a chapter for a topic, see How can I add a chapter and/or new page(s) in Guidelines?
If you have created a chapter for a topic, click into the text bar of the chapter title until a black box containing three icons pops up. Select the gear icon on the far right of the black box.
This will bring you to the settings page where you can edit the chapter title, choose its display settings, add content, and add an image which will serve as the page's tile.
To add an image to serve as the page's tile, click on "Upload image" and choose the image file you wish to use. Click "Save" before moving on to edit another chapter.