There's no way I as a user can prevent other usage of my photos after I give them permission to access them all. I therefore have to trust Spotify that they won't abuse their access to my (and all the other people who install the app)'s photos.
Now I understand you trust them completely, and that's your right, I don't trust them at all especially considering their strong ties with Facebook.
Regarding their public 'Sorry' posted this afternoon: https://news.spotify.com/us/2015/08/21/sorry-2/ it's still vague. They state "Photos: We will never access your photos without explicit permission", but there's no state possible where the user can use the app and _not_ have given permission, as the app is only installable if the user gives access to the photos through the Android access rights at install time. After that the app already has access to the photos, and Spotify can state that they won't access the photos 'without explicit permission' but that's moot: to not give them explicit permission I have just 1 option: not install the app (otherwise I have to, see above). Which is a bit of a shit option, really, if you pay for the service (like I did) and want to listen to music.