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 😊