Where is iTunes artwork saved and located?

I have spent a lot of time adding artwork to my albums on iTunes. Many were not available through the iTunes "Get Album Artwork" command so I had to find the images from the web or even scan the CD covers locally. Given all this work I do not want to lose them! My intention is to back up my iTunes folder in full to an external disk. I see that this folder has a sub folder Album Artwork, but will this include all my artwork or just the ones found from the "Get Album Artwork" command. Is all the artwork saved in one location or are the ones I found myself stored somewhere else?

Any help much appreciated.