Copyright is a set of exclusive rights granted to you, the creator of an original image or artwork. Copyright includes the right for you to print, copy, distribute and adapt your work. So when you create an image or an artwork then copyright automatically exists in that work and it belongs to you. Images are protected for the lifetime of the creator, e.g. the artist or photographer, and for 70 years after his or her death.
Copyright ownership is separate from ownership of an original artwork. If the work is uncommissioned, then the artist owns the copyright automatically even if the original is sold. The copyright may be sold or assigned as a separate undertaking. Only the copyright holder has the right to reproduce the work. If you have any doubt, seek professional legal advice.
You should scan or photograph your original artwork before it is sold as the new owner is under no obligation to allow access once the sale is complete. Remember, it is only polite to inform a purchaser if you intend to reproduce an original that they have just purchased.