Where does Apple get the Album Artwork?

I'm in the process of going through my iTunes music library and updating some of the missing Album Artwork. At first I started searching the web to find artwork. But the quality varied widely (mostly bad). So, since 99% of my music is stuff I've ripped myself from my own CDs, I'm now scanning the actual CD liner artwork in Photoshop and then manually importing each image into iTunes. I'm using an image size of 500x500 @150dpi to create each missing Album Artwork file so the quality is excellent.

Anyway, during this process, I also noted a fair number of the covers (Album Artwork I pulled down from the iTunes store) are of fairly low -to sometimes very poor- quality. Some covers are "odd" sizes (not square).

So, I'm wondering.... Where does Apple GET the Album Artwork? Does it have its own "search engines" that go out and search the web? Is there someone sitting at a desk in Cupertino with a scanner and a huge stack of CDs? Are they getting some of them directly from the record companies (much of the downloaded artwork is of very nice quality)?

And, perhaps most importantly, do WE automatically share our Artwork with the iTunes store? Does iTunes track which Artwork I've manually updated and then share (upload) that to the iTunes store for use by others?

Just curious how it all works!

Mark