previous post concerning map data); what about the painting itself? (And let's simplify matters even further by only considering the UK.)
If an artist sells a painting, who owns the copyright?
The artist does.
I was under the incorrect impression that the copyright went with the physical painting; for this reason, I was agonising over printing copies of sold work in books or on cards, particularly in cases where I didn't know the purchaser personally. Turns out I didn't need to worry - although it does seem courteous to inform the owner of a work if a print is to be issued.
There are exceptions; copyright can be transferred by agreement, and employers own the copyright for work done under their employ. This doesn't include freelance work or commissions, though.
Reference: The UK Copyright Service