The artist still owns the copyright. Payment by itself does not transfer copyright. To do that the artist needs to explicitly sign away those rights. This happens in employment all the time. Part of the paperwork you sign is about transferring over the copyrights from yourself to the company.
I highly recommend you check your own paperwork to see exactly how much this covers, since some states allow contracts that cover everything you make at any time. California has a specific law that limits these contracts to only works done on company equipment and on company time. Your state might be different.
I highly recommend you check your own paperwork to see exactly how much this covers, since some states allow contracts that cover everything you make at any time. California has a specific law that limits these contracts to only works done on company equipment and on company time. Your state might be different.