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

[merlem]

Maybe this can help you?
http://scikit-image.org/docs/dev/api/ski...d_contours

Edit: some 'quick and dirty' solution based on what they describe in scikit:

import PIL
import PIL.Image

ima = PIL.Image.open("Bild90.tif")
imsize = ima.size
boundary = []
for imapp in range(imsize[0] -1):
    for imapix in range(imsize[1] -1):
        pix1 = ima.getpixel((imapp, imapix))
        pix2 = ima.getpixel((imapp, imapix +1))
        pix3 = ima.getpixel((imapp + 1, imapix))
        pix4 = ima.getpixel((imapp + 1, imapix +1))
        if not (pix1 == pix2 == pix3 == pix4):
            boundary.append((imapp, imapix))

imac = ima.copy()
imacopy = imac.convert("L")
for pixel in boundary:
    imacopy.putpixel(pixel, 255)

imacopy.show()

If the boundary needs to be only one pixel wide, one has to refine it a bit further.