Draw multiple lines on image

Question

Hi!
i have an image(say captcha image).the background of image is transparent.  i want to create an app in c# that check the color of  pixels vertically from starting of the image(say color checker).so a point will come where the color checker will reach to between first two characters of image so definitely the pixel colm will be empty at that point, so the app saves it a an png image up to that point and start checking it again futher doing so and saving characters one by one.
can somebody tells me how this will be done..?
thanks in advance