Char型だとダメで、Cuchar型の配列だと、writeで配列を指定するだけでOK。
function saveppm(fname, w, h, img)
open(fname, "w") do fp
write(fp, "P6\n");
write(fp, "$w $h\n");
write(fp, "255\n");
write(fp, img)
end
end
img = Array(Cuchar,WIDTH * HEIGHT * 3)
for i = 1:length(img)
img[i]=Cuchar(round(Int,rand()*255))
end
saveppm("image.ppm", WIDTH, HEIGHT, img)