From the perspective of a Publishing House, first-time authors usually have an eight percent portion of the profit for royalties. ...if they ever see it. Second-time authors can or should receive ten percent. Above that, anything to fifteen percent can be considered normal.
First-time artists...? I'll say two to five. Second-time, five to ten. ...and might not rise higher than that. Twelve at the most, maybe?
You as the publisher, can do what-ever you want. And if you are kind ( and prompt! can not stress this enough! ) with those residuals, the artists should be kind and consistent for you.
Some artists like the idea of a publisher buying the art rather than receiving residuals / royalties. However, residuals allow a non-famous artist to receive consistent payment, even if small.