When I rip music to mp3's to a folder with album name. Then look on amazon or google search albumn name and click on images. I stick with 300x300 size seems right for many different devices. And download the jpg cover into the folder with tracks.
I have also been having problems with my album art showing up on my Nook HD+. I got it to work on some by adding the art work with WinAmp then adding the jpg to the folder with the mp3s. I also noticed that once you add a folder of music to your nook and the artwork doesn't show up it will never work unless you rename the folder and reinstall the mp3s with the jpg attached to it (by using WinAmp). I've wasted hours of time with this and it is really frustrating me. Why don't they just have a tab to add new album art like every other music player has. Itunes and WinAmp show all my album covers, why doesn't my Nook Music player show them? I seriously feel like buying an ipad instead just because of this.