This is a image storage space for things like the Banner image on the Invitations page.
To use images in code you have to store the image within squarespace, not externally on something like Dropbox.
to do this you write out the title of whatever the image is, such as 'Invitation'
Then you highlight it and click 'link'.
Then choose the tab 'file' and add your image of choice.
This will then create a link you can copy to add to the Banner code.