Lots of backgrounds from 1.5 are hidden away in the Biohazard 2 trial, from what I've read. Good job for the programmer/designer who put those in! Some relevant links (courtesy of Redfox McBeth, of the BrainBread mod forums):
Topic where this was announced
http://www.exploding-dog.com/re15.rar - the backgrounds, saved as JPGs (decent quality)
Finally, Bioflames, the 1.5-centric site http://www.bioflames.com/