Quote Originally Posted by the tree
Not if it's a visual watermark, such as a big black signature.
I've already stated that, if you didn't catch it. Anything above 128 for the alpha is what you need. Having it as a solid black, however, is the best way. Adding a bunch of noise for the transparency (make the opacity of each pixel varied for the watermark) makes removing the watermark much more difficult. Also, save your watermarked image as as JPG as JPG changes the colors of pixels some to enhance compression (it's a lossy compression). The lower the quality, the harder it is to remove the water mark as you'd also have to reverse the JPG algorithm and apply the algorithm to remove the watermark. So, watermark your image and JPG it.