4:
Once you are on the
'Edit Frame'
page, click on the
'Source Button'.
Scroll to the top where you can see:
background-image: url (https://.secure......image-name.jpg)
Here, you need to replace the contents inside the brackets with your new image URL. If you don't know this,
here's how to get the image URL.
Press
'Save'
to close the Source Editor.