How to extract (x,y) coordinates of the boundary of a .tif image?

[merlem]

First, the RGB-type for the human pattern recognition part:

rgbimage = imac.convert("RGB")
areacol = rgbimage.getpixel((imsize[0] - 1, imsize[1] - 1))

for provenpix in provenpixels:
    rgbimage.putpixel(provenpix, (0, 200, 200))
for stillunprovpix in remainingunclear:
    rgbimage.putpixel(stillunprovpix, (200, 200, 0))
  
for stillunprovpix in remainingunclear:
    rgbimage.putpixel(stillunprovpix, (255, 0, 0))
    rgbimage.show()
    print "Is this a border pixel? It has the coordinates: ", stillunprovpix
    while True: 
        pixelinfo = raw_input("yes (y) or no (n)? ")
        if pixelinfo in ("y", "ye", "yes"):
            provenpixels.append(stillunprovpix)
            rgbimage.putpixel(stillunprovpix, (0, 125, 125))
            break
        elif pixelinfo in ("n", "no"):
            rgbimage.putpixel(stillunprovpix, areacol)            # remove mark
            break

Second...As I said, the code does mostly what you propose.
Only this part I don't understand:

 it probably depends on what the boundary is to be used for and how it will be used to make these decisions.

If "1-Pixel-border" is only what I assume it to be, there is no difference depending on the usage. It's just a question of "is this pixel one that makes the demarcation between the both colour regions?" 

And if you like to think about patterns: How would you solve a region like this in universal rules? I was not able to get a "full" solution.
Bit of ASCII art, "x" is black (background color) and "-" is blue (area colour):

xxxxxxxxxx-
xxxxxxx----
---xxxxx---
--xxxxxxx--
--xxxxxxx--
---xxxxxx--
-----------

With checking a maximum of 8 neighbours, I was able to prove the border state of a couple of pixels, shown here as "o":

xxxxxxxxxxo
xxxxxxxoo--
oooxxxxxo--
--xxxxxxx--
--xxxxxxxo-
---oxxxxx--
----oooo---

Switching to "o" for the pixels with unproven border state:

xxxxxxxxxx-
xxxxxxx--o-
---xxxxx---
--oxxxxxxo-
--oxxxxxx--
---xxxxxxo-
--------o--

I fear one has to widen up the range of neighbours to a total of at least 15 for those...