I upgraded to 4.7 from 4.6.0 and this problem happened to me. it was not there before. I used the upgrade scripts. All of my banners showed as broken. And I got the no permission message on the vendor (site vendor that is) logo and the individual banner images. I resynced the site file listing as it was blank (no idea why, probabably the issue above about not having a root folder listing). This populated the list and I could find the banner files. This did NOT fix the problem with the display though.
Looked in the DB and the file ids do not match the file ids in the files table for the images. I'm assuming after reading this thread that my sync of the site file system likely recreated all of the files (with new ids), removed the old ones, and therefore didn't fix my problem.
I had to change all of the ids of the banners to the new ids and that fixed the problem. But I'm guessing that had I done the root folder entry (and probably all other fodler entries that might have been missing) that would have fixed the problem. If not, hopefully my experience helps.
Basically it looks like the upgrade may have cleared out all of the folder and/or file entries on the portal.
Dax