a month ago i noticed that spotify crashes when i try to add or change a playlist cover. i've tried everything and the problem is still unsolved, and i know a lot of people that use spotify desktop for linux have the same problem. what i am going to write here is not a resolution, but an alternative.
there is this site that allows you to change you to edit the information of your playlists, here's the url: (https://change-playlist-image-spotify.virock.org). all you have to do is log in with your spotify account, and you'll be able to change the images of your playlists without a problem.
i'm like the guy in baby driver: i create playlists for everything. not being able to change my playlists' images was a little bit upsetting, but that site came in handy.
i hope this post was helpful to you!