A "header photo" is an image or picture that appears at the top of a webpage, social media profile, or document. It is used to make the page more visually appealing or to provide information about the content below it. Full definition
Facebook continues to modify its News Feed to include larger and more prominently displayed images and videos, Pinterest is now the fourth most popular social networking site, and Twitter now pulls pictures and videos right into its timeline — and has used its recent profile pages update to allow for larger header photos and profile pictures. (blog.hubspot.com)
Not only did Collins wear a Michigan backpack to a recruiting event, but the Pinson (AL) native is now sporting the following photo on the left as his Twitter header photo: (diehardsport.com)