Putting myself in their shoes, I am not sure I would agree to paying you a percentage of sales and also be restricted to the number of images I could use. From another point of view, once the product is established will they need your image? If they dont and they have reached the limit of images they can use, why should they pay another "Licence Fee"? They could actually afford a drop in sales.
An annual option/retainer fee for future products might tempt them.