how to scan black pixel on image document

hello everyone.. how can i scan the black pixel in left of image document and store it. i want to make a line after i scan the black pixel to get the coordinate. i want to draw a line in the left of image document. see the example of image here..

http://i1259.photobucket.com/albums/ii548/msaiful89/sc0003.jpg

i want to draw a red line as in image given. please help me to do this