Bitonal (black and white, no gray) images should be matched correctly. It is possible that there is some drop in accuracy due to the loss of information in bitonal images. I am not sure why you need B&W image matching. Most scanners return grayscale images. If you are trying to reduce image size, it is better to use WSQ compression. If you have a bitonal image that looks OK to the eye but isn't recognized by our algorithm, send us a sample and we will have a look at it.