If the artist is posting personal work, then it is covered by copyright automatically per the Berne Convention.

Copyright exists to incentivize authors, artists, scientists, etc. to create original works by providing a temporary monopoly.

If an artist works with a client, the legal stipulations are going to be different for each commissioned work, generally.

Depending on the client, they may be better able to protect an artists work than the artist themselves, and with credit...