What's special about this image?

Question

There's something very special hidden in this image. You will definitely know when you find it.

(I know Stack Exchange lossily compresses images but I've checked and no pixels have been altered.)

Finding traces of the secret might not be that hard. The ideal answer will point to the Stack Exchange question that I took data from to help generate this image (and possibly point to another that can help "decode" it).

Hint:

I can reveal the secret with this 20 line Python program. I've replaced the most telling bits with ???:

from PIL import Image
i = Image.open('hidden.png')
d = i.load()
x, y = ???, ???
while ???:
    oldX, oldY = x, y
    if ???:
        ???
    elif ???:
        ???
    elif ???:
        ???
    else:
        ???
    d[oldX, oldY] = (0, 0, 0)
for x in range(i.size[0]):
    for y in range(i.size[1]):
        if ???:
            d[x, y] = (255, 255, 255)
i.save('revealed.png')

Answer 1

This image represents instructions for a snake that fills the visited pixels with black paint.

• Red color means "go right"
• Aqua color means "go left"
• Blue color means "go up"
• Green color means "go down"

The snake starts at the center of the image. (Edit: looks like it can start anywhere but the borders. The images below were produced using center as the starting point.)
The snake stops as soon as it proceeds out of the image boundaries.
After we're done, we fill remaining unvisited pixels with white paint.

---

What is interesting is what happens if the snake visits a block that was already visited: it turns out that no matter if we go left, right, down or up, we still see the result pretty clearly.

Going left: Going down: Going right: Going up:

---

#!/bin/python
from PIL import Image

colors = [(255, 0, 0), (0, 255, 255), (127, 0, 255), (127, 255, 0)]

i = Image.open('hidden.png')
d = i.load()
w, h = i.size
x, y = w >> 1, h >> 1
while x >= 0 and y >= 0 and x < w and y < h:
    oldX, oldY = x, y
    if d[x,y] == colors[0]:
        x += 1
    elif d[x,y] == colors1:
        x -= 1
    elif d[x,y] == colors2:
        y += 1
    elif d[x,y] == colors3:
        y -= 1
    elif d[x,y] == (0,0,0):
        y -= 1 #or whatever
    else:
        raise Exception('Bad color: ' + str(d[x,y]))
    d[oldX, oldY] = (0, 0, 0)

for x in range(w):
    for y in range(h):
        if d[x, y] != (0, 0, 0):
            d[x, y] = (255, 255, 255)
i.save('revealed.png')

Answer 2

Putting it through a magic eye finder gives a faint image of a smiling face, perhaps the Mona Lisa.