This one time after I installed a custom ROM, I somehow ended up getting some random image applied as an album art on pretty much all of my songs. Later on when I switched to a different rom and opened up my music player, I realised that the album art image was still there.
The reason for the album art not going away is because we have it got cached inside a file stored in our internal storage under the Android data folder. All you have to do is delete that file and that pesky album art image should be gone from all of your songs.
Step 1. So let us start by finding that folder. Open up your file manager app and select your internal storage (the file is never stored on your external sdcard) and navigate to Android -> data. Inside it, you should see a folder named 'com.android.providers.media'.
Step 2. Open it. There should be a subfolder inside it called 'albumthumbs'. Open that folder as well and you should see a file named after random numbers.
Step 3. Simply delete that file and close your file manager app. The next time you open up your music player your songs will be free from album art.
Note: It is possible that the image may re-appear after you change your rom again. In that case, simply re-do the steps from this post.