At an old temp job I had, my manager, much older than me, habitually photocopied faxes as they were received precisely because the thermal paper fades over time. Even stored well, in the dark, in a dry box, they'll gradually fade over time. Leaving them exposed to the light will fade them really quickly.
I think you would need to scan the images and have them printed onto proper photo paper if you wanted to keep them. Keeping them in the dark slows the fading process, but that is all
