Memories lets you customise many aspects of your Timeline and one of which is your background photo/image. You can change your Memories banner by uploading a new image.
Step 1 - Click the "camera Icon" button at the top of your page, this will allow you to change the background photo of your Memories Account.
Step 2 - You can edit the image during this process
After selecting image, you have the option to edit, i.e. crop, circle and or rotate it.
Step 3 - Select 'Upload' to finally save the changes made.