Spotify is one of the most popular music streaming services available and allows you a great degree of control over the way you experience your music. Included in that is the album art for your playlists. While not an option ingrained in the mobile app itself, changing a playlist's cover image on your phone can still be done.
Unfortunately, this trick is only available to iPhone users right now, as the Android version of Spotify doesn't allow you to edit playlists beyond a simple name change. You're also limited to changing the playlist's picture to an album cover that's already in the playlist or one you add to the playlist.
To make it a different picture entirely, like a mishmash of album art or a random photo that invokes a particular emotion, you can do so on Spotify for desktop. Open up the playlist on the desktop app, click on the playlist cover, then select "Choose Image" when hovering over the cover art box and follow the prompts. If you just want a quick fix on your iPhone, use the steps below to change the cover art.
Since you can only switch the playlist image from the iOS app to another song's cover art in the playlist, you'll want to make sure the song with the cover you want is in there. If it is, great, skip down to the next step. If not ...
Find the song, then once you have it in front of you, tap the ellipsis (•••) icon to the right of it and hit "Add to Playlist." When prompted, choose the playlist you want to change the picture for or select "New Playlist" to create one from scratch.
To get to your Playlists, tap on the "Your Library" tab, and hit "Playlists" at the top. All of your playlists will appear here, so tap on whichever one you want to change the picture for.
From here, tap the ellipsis (•••) icon in the top-right corner, then hit "Edit." Red circles will appear to the left of your songs (tap them to delete music), and three vertical lines will appear to the right of them.
Find the song on the album that you want to make the picture for your playlist and drag it (touching the symbol) all the way to the top of the playlist. Now, the playlist picture will change automatically. Hit "Done" in the top right to save your changes.
Note that if you added a custom playlist cover from your desktop computer, rearranging the songs in a playlist will not change the playlist cover on your iPhone — it will stay the same as whatever you set on the desktop version.