These instructions are for Retina 4.0 and up. If you are on an older theme version, please update your theme.
You can make use of the built-in collection.banner template to incorporate a full-width collection banner. To do this, go to Admin > Products > Collections and open the collection to which would you like to apply a full-width banner image. Select the image you would like to display as the banner using the Collection Image setting. In the bottom right-hand corner of the edit screen, select collection.banner from the template suffix selector, and save.
Then, before configuring the collection.banner settings, be sure to add a method to navigate to the collection page from your home page, by adding a link to your main menu (Admin > Online Store > Navigation).
Next, go to Admin > Online Store > Themes > Customize. Within the Customize settings editor, navigate to the collection page in the preview window on the right, to access the Collection banner Sections settings on the left. Here, you can configure the settings that will apply to this template specifically.