Set of classic test images :

airplane.bmp or airplane.ppm : color 512x512

baboon.bmp or baboon.ppm : color 500x480

barbara.bmp or barbara.ppm : color 720x576

barbara.pgm : grey 512x512

boats.bmp or boats.pgm : grey 720x576

BoatsColor.bmp or Boats.ppm : color 787x576

goldhill.bmp or goldhill.ppm : color 720x576

lenna.bmp or lenna.ppm : color 512x512

pepper.bmp or pepper.ppm : color 512x512