This is the first site that collects the almost all standard gray images which persons in the image compression field often used. Hope these images can do help your research.

barb.pgm	couple.pgm	crowd.pgm	girl.pgm
hustler.pgm	lena.pgm	man.pgm	marie.pgm
tiffany.pgm	woman.pgm	zelda.pgm	baboon.pgm
boat.pgm	bridge.pgm	flower.pgm	goldhill.pgm
lake.pgm	peppers.pgm	plane.pgm