Navigate to Content > Pages > click to edit the Index page > Header Image
The Header Image for the home page offers the ability to display a single static full-width image.
Image Dimensions: Header Images should be at a ratio of 288 : 127 with the recommended size of 1440px by 635px (for retina images you can use 2,880px by 1,270px).
Resolution: 72 px/inch - Save for Web
Format: .JPG or .PNG
Correctly size your Header Images before uploading them into the Header Image.
Help Site - How To Resize And Crop Your Images Online