Noted Star Wars author J.W. Rinzler provides the following post on the Official Star Wars Blog on the differences between a Ralph McQuarrie concept painting and a piece of Lucasfilm licensed artwork.
Check out the intro below and then take the source link for the rest of the blog post.
The easy answer is, look for an awe inspiring composition, a refined color palette, dynamic character poses, and original ideas. But my job in researching the Making of Return of the Jedi was to differentiate between a McQuarrie production illustration and his licensed artwork. Not so easy as it might sound, as they’re all stored together in the archives.
And not so easy because, starting with Empire, Ralph did 1) concept artwork to come up with solid visuals for George Lucas’s ideas and consecutive drafts of the script; and 2) licensed artwork intended to supplement those concepts, based on filmed portions of the film, which would be published in a portfolio edition. Some fans get confused because of the latter group, thinking that director Irvin Kershner and/or production designer Norman Reynolds were copying verbatim Ralph’s artwork—in fact the opposite was true. If the artwork and film moment are identical, Ralph was embellishing film stills in illustrated form, or taking approved designs and creating new illustrations after the fact, though they are nevertheless wonderful.