This app was mentioned in 5 comments, with an average of 1.20 upvotes
I use this https://play.google.com/store/apps/details?id=mobi.infolife.itag
Make sure if you add a space to the end of the artist name that you do the same for every single one that fits the artist. After some trial and error you will get it.
> The safe for work artwork is in the folder which I transferred to my phone, but for some reason the app displays the NSFW artwork, despite it not being in the folder or on my phone anywhere.
That isn't how MP3s with album art work. Music apps read the ID3 tag which gives all information about that MP3 (header, title, artist, album, year, comment, track, genre and album art).
You can use an app like iTag to change the album art. Personally, I use EasyTAG to set all the ID3 tags before transferring them to my phone/media server.