Just a quick selection of images randomly-generated by the following 10 lines or so of Python.

import sys
import numpy as np
from PIL import Image

try:
    filename = sys.argv[1]
except IndexError:
    filename = 'img.png'

rshift = 3
width, height = 600, 450
arr = np.ones((height, width, 3)) * 128

for y in range(1,height):
    arr[y,0] = arr[y-1,0] + np.random.randint(-rshift, rshift+1, size=3)

for x in range(1, width):
    for y in range(1,height-1):
        arr[y,x] = ((arr[y-1, x-1] + arr[y,x-1] + arr[y+1,x-1])/3 +
                            np.random.randint(-rshift, rshift+1, size=3))

im = Image.fromarray(arr.astype(np.uint8)).convert('RGBA')
im.save(filename)