An image for the unified model of AGN
I agree with your supervisor: If your paper is going to a peer-reviewed journal, you should cite peer-reviewed material. I also agree with you that the NASA version is clearer than the original figure, but if you're only going to refer to the image — i.e. not show a reproduction — I'm sure the reader can figure it out.

Alternatively, you could create your own (color) version with the annotations that are relevant for your work, and then write something like "…based on the schematic diagram of Urry &amp; Padovani (1995)".