New Post: create valid barcode image

I've changed the font size to 72 and I noticed that there are some grey shades around some bars and I think this is the reason the code is not recognized (it looks exactly the same with the one that is OK, except those grey shades; see here - try 400% zoom). Can you please tell me how to get rid of gray lines and how can I reduce the bit depth to 1?
Thank you,
Alex