You have most likely getting a RGB image and converting it into a greyscale and converting it into a binary image format. You would have done this without assigning buffer to the functions (IMAQ create to img dst).
the image you see (black) is still a valid one for processing it is just that the buffer is getting over written. To view it right click on the image choose palette and binary and you will most likely see the image. assign proper imaq create to view the image.
