Moreover , I suggest files are served by php program , not by apache server.
The image I uploaded are saved to the three destinations:
https://www.ubbcentral.com/forums/gallery/48/thumbs/592-CRW_8476s2.jpg
https://www.ubbcentral.com/forums/gallery/48/medium/592-CRW_8476s2.jpg
https://www.ubbcentral.com/forums/gallery/48/full/592-CRW_8476s2.jpg

There files seem served by apache , not a php 'filter' .
If it is served by php program , it should be able to add more authentication to access these images.

English is not my native language. I try my best to express my thought precisely. I hope you understand what I mean. If any misunderstanding results from culture gaps, I apologize first.