Digital images are easy to steal, particularly when they are uploaded onto the internet. A right-click on a mouse is all that is required to take a copy of an image in many cases. For snaps and photographs of little importance, this may not be too serious. However, for serious photographers who may have invested a lot of money and time in obtaining high-quality images, the theft of their work may threaten their livelihood.

Many amateur photographers take the view that only small, low-resolution images should be uploaded to the internet. This denies those who steal images anything other than low-quality images which are of little practical use. A better solution is to watermark all images which have any significant value.

Watermarks may be divided into two broad categories - visible and invisible. The visible watermark is by far the most widely used because it is cheap and simple to achieve, can be seen by people looking for images, and renders the marked pictures effectively useless. The disadvantage of visible watermarks is that they work by embossing part of an image, and consequently damage it aesthetically. Visible watermarks can also be applied to a large batch of images at one time using free or relatively inexpensive software. A simple visible watermarking procedure is described under "basic watermark".

Invisible watermarks are much more sophisticated and, as their name suggests, cannot be detected by the eye. High-quality images can therefore be viewed on the internet without the need to visually change them. This is consequently a much better approach for those who want the beauty of their work to be admired. Of course those who would take copies of such images are also unable to see the watermarking, and may therefore use the image for their own purposes. It is therefore necessary to have a tracking mechanism in place which detects invisible watermarks in published images. This makes the process of invisible watermarking much more complicated and expensive than its visible counterpart.