the copyright is always yours, well after you die. even if you sell the image, the original - the copyright is yours. its not transferred to anyone. and if someone wanted to buy the rights to use it, they don't own the copyright unless you sell that to them. selling the copyright can be sold at many times more than what you would sell a regular license as. once it's their copyright, you would have to erase it from everywhere.
but you don't give the copyright away and it will always be yours. now if you did a private commission of a person, they may have a say if you can sell it some place else.
---Mike Savad
MikeSavad.com