An old chestnut, I know. I have a corporate friend who was telling me
about a buyout deal he negotiated with his designer to buy the rights
to company-related artwork he hired this person to create.
The point that was in question in my mind is this: do you as the
creator in fact have the right to hold onto - or sell if you choose -
the original work (source files) even after you were paid for the
design gig or does the fact that you were hired to design the piece
in the first place automatically grant the client rights to all files
and give them free reign to do what they like with them? The designer
in the above example negotiated a very sweet deal indeed.
If any portion of the artwork was supplied by the client, even though used in a way uninteded, the client still owns the rights.
If you supply any wholly original portion of the artwork, such as a background photo, that is not inherently related to client project even though used as part of it would still be yours. You would just be selling rights to use it as part of the artwork. This would only apply to things you alone have supplied as opposed to purchasing to use.
Original designs such as logos is a bit of a grey area. I believe the original designs are property of designer, but you wave all rights to their use or copyright to the company who owns the brand.