Header images (and all other design settings) can be added to your bundle page by navigating to a bundle's Design Editor. From there, you will find the option to upload a Header image.

Navigate to your Bundles, click on the bundle you'd like to edit (or create a new one) and click on the 'Design editor' in the top right of your bundle:

Select a template that you think would work for you, and you'll be taken to the Design Editor, where you can add all of your content - including a header image.