A contract is mandatory for any commission. Write down every detail in the contract, such as the size and media of the artwork, your compensation, due dates for payments, who is responsible for framing, installation, delivery of the artwork, etc. If your client balks at signing a contract, WALK AWAY.
The artist agreement is between two parties who agree to work on specific projects and it governs their relations inter se. It is necessary to have a written agreement with definitive terms to avoid any disputes in the future between the parties.
It is illegal. Once any piece of work is created, it is automatically copyrighted. This includes art, photography, music, etc. You have to have the creator's written permission before you use it anyway at all. If you want more information, you can search for copyright infringement.
Contracts like an artist agreement help avoid miscommunications and confusion over the rights and responsibilities of both parties involved. Artist agreements typically include availability expectations, performance markers, and licensing or ownership rights of the works created during the partnership.
Like its name suggests, an exclusive contract is one that restricts a party from providing the same goods/services to others for a specified period of time. This can also be applied in the context where a company obtains exclusive rights and agrees to do business exclusively with another company.