Black and White watermarks are very popular because they often stand out on your photographs more than your standard logo watermark.
Please note it is not always possible to turn your existing logo into a watermark, another alternative would be a text watermark, so please email before purchasing this item to check it can be done.
PNG: This is a file suitable for web use. It has a transparent background which makes it great for watermarking. You can use design software such as Photoshop or Picsasrt to overlay your logo onto your photograph.