Under certain conditions, licensing personal works is negotiable.
Illustration work is a shared copyright with the publishers, therefore licensing of those images should be an inquiry with the creators of the illustrated content.
Conditions for personal licensing are entered into only by their prospective benefit. This means no exclusive contracts or any contract that restricts artist ownership of the original artwork or print production.
An exception for exclusivity might be made for a single image created expressly for that purpose or if it is for a product that I do not produce personally. If the image is created for that express purpose, there must be compensation for the time and production of the original, although not neccessarily the purchase of the original art, as well as the purchase of the rights to license exclusively.
If wishing to license exclusive rights to a particular form of product there must be an initial license purchase as well as a percentage of sales.
For open non-exclusive licenses that in no way inhibit artist rights to the artwork or the selling of artist produced products, a percentage of sales is a viable option unless there is labor involved, which will require compensation for that labor.