How do I free up space (delete things) taken up by my iPad's Photo Library?
On the newer iOS versions, Apple initiated a delayed trash. This means that things are only truly deleted after they've been in the trash for 30 days.

To force things to delete, go to the photos app, then choose albums in the bottom right corner. Scroll down until you find the "recently deleted" album and click on it. Then click select and choose whether you want to delete or recover either all or just a few of the photos.