As the author, it’s important to own the copyright on your illustrations. You want to be able to remix them yourself and use them for future purposes (promos, merch, whatever), without having to ask permission.
Specify this in the contract. This will cost a bit more, but it’s worth it.
Also allow the artist to retain the right to display them in their portfolio. This helps them and can be good for you as well if they have social media reach.
image: Photograph of Greg Wilson taken by Ian Tilton. It was used as artwork for the Credit To the Edit album compilation by Greg Wilson released on Tirk Records. https://commons.wikimedia.org/wiki/File:Greg_Wilson_C2TE.jpg https://creativecommons.org/licenses/by-sa/3.0/deed.en
