Hey @sws,
Thanks for this question, and welcome to the Prismic Community 
In the past, I've seen other users make their own image library using Integration FIelds. You could use any image hosting provider you want, like Imgix or Cloudinary, and write an API integration to send your images to a Prismic Integration Field. Then the content editors would select images from the Integration Field, rather than from the Image library. The user experience wouldn't be as good, but it would allow for more advanced control of your image delivery.
Here are the docs for Integration Fields.
Let me know if you have more questions!
Sam