"Artists Beware"

Hello,

I am a buyer attempting to commission some artwork for a novel I've been working on, which is now finally nearing completion.

The novel is split into eight chapters, and I want each chapter to have a little bust-type sketch of one of the eight main characters on the chapter title page. In fact, I have already commissioned a set of busts for the cast for this exact purpose. They are already completed and can be seen here and here and they are lovely. Of course, I dramatically underestimated how early in the process I had this idea versus how long it actually takes to write a novel; these were made three years ago, and the artist has expressed interest in redoing them with the improvements and such she has made in her art since then. I personally thought they looked outstanding already, but if she wants to make them even better, then great! She was a pleasure to work with last time, and I would be delighted to do so again.

The problem that both of us are running into is that neither of us knows what the standard guidelines are for small-scale commercial work. When this novel is done, I intend to put it on some small sale service, such as selling it on the Amazon Kindle marketplace for $2 or so. (If it randomly catches fire and becomes such a smash hit that big-name publishers come in and want to take it, awesome, but I'm honestly not expecting that as a likely outcome.) Therefore, I would technically be profiting off the artist's work, which brings up all the licensing questions and such that neither of us really know. Unfortunately, the problem would persist even if I decided not to get those new updated versions after all; whether I'm using the old ones or the new ones, I'm still using her work in a novel I'm selling. (Besides, I really do want to see what updated versions of them would look like.)

She has asked around, but has not found anything yet. Her exact words from a note I just received:
"[W]hen I posed some questions to friends and groups that supposedly knew, they instead kept referring to larger, much more commercial endeavors than what you are wanting, which is much smaller in scale; I didn't get anything concrete for something specifically like this."

She said she would keep looking, but encouraged me to join in the search in the mean time. Which seems fair; it shouldn't be entirely on her to figure this out if I can help. Of course, the only place I can think to ask is here, so here we are. Ideally, how should my intention to put this novel on something like the Kindle marketplace affect the pricing and permissions and such for commissioning her?