It seems like everything has been suggested, but you might also try going to TM disk > ~/Photos > Get Info > unlock, add your own user account, and in the little menu choose 'apply to all items' and let it finish so that your account
definitely has access to everything in there.
If that doesn't work, maybe apply your permissions to the whole backup folder, or the whole disk. I don't think any harm can come from this.
If that still doesn't work, have you tried copying a new user account? It's usually one of the first things mentioned but I didn't see it this time.
Try applying that account to the library's/folder's/disk's permissions.