To update the banner image of portal just follow the below steps
Step 1 Go to D365 CRM – Portals – Web Files
Step 2 Search for “homehero.jpg” and open the resulted record
Step 3 Go to the notes section and replace the attachment named homehero.jpg with your jpg image
Note : Make sure the name of the update attachment is same i.e. “homehero.jpg”
Refresh your portal and you will see the updated banner
Happy CRMing 😊
SOURCE : mscrm.com