Hi, Sandor!
First of all, congratulations on your visual novel engine! It looks super awesome.
Regarding your inquiry, it's a complicated issue.
When you put graphical assets in your engine's files (or release them on your website), to let others use them creatively, this is sharing with third parties. Unfortunately, it can't be done with the single user's licence.
However, I can create graphical assets especially for your engine, and sell them to you with all rights. In this scenario, you could share these assets, as you'd be the sole owner of all rights to them.
Let me know if it's something you'd be interested in.
Rachel