The Home Page offers 3 display options:
1. Header Image: a static full-width image (see instructions below)
2. Rotator: rotating slide images (see instructions here)
3. Header Background Video: looping video (see instructions here)
By default this theme has been setup using the Header Background Video option. You can select which option to use on your home page by editing the Index Page. (Content > Pages > click to edit the Index page)
To add a Header Image go to Content > Pages > INDEX > add the image in the "Header Image" field.
Image Dimensions: This image should be at a ratio of 16:9 with the recommended size of 1,600px by 900px (for retina images you can use 3,200px by 1,800px).
Resolution: 72 px/inch - Save for Web
Format: .JPG or .PNG
Correctly size your Header Images before uploading them into the Homepage Rotator.
Help Site - How To Resize And Crop Your Images Online