The artwork selectors vanished two builds ago. I, and presumably others, have submitted feedback & bug reports. Maybe next time...
Hey, I was really annoyed by this too, and I have also used Bug Reporter, hoping to get response from the dev team, but I don't have much hope.
My theory is that Apple dropped this function in order to reduce album art to just one picture, because it might cause to much trouble with iTunes Match.
I think it is no coincidence that this feature was removed about the same moment the iTunes Match service was launched.
I could be wrong, but as I understand, there's several methods for adding artwork, the only way to embed it into the files is to add it through the "get info" pane.
If people embed the artwork in the file itsself, this alters the file, making the file longer. Maybe Apple doesn't want too many duplicate files with variations of artwork on its Match servers?
Does that sound plausible? I know it's a speculation, just thinking for any reason why they would remove a useful feature.
I'm really angry about it. I've spend uncountable hours of work since 2003 adding artwork to my library.