The short answer is that you cannot. The only certain means of making sure your work cannot be copied is to keep it off the public web, and never allow any copies into the hands of someone who might publish anywhere. Anything that gets printed is quite likely to end up on the internet.

Some people advocate technical tricks such as slicing your image into lots of small pieces and having the browser reassemble the parts for display. This certainly makes 'saving'  from a browser hard work, as there are many parts that need to be saved. But a simple screen grab defeats all that completely.

The same applies to web authoring techniques that place the image in the background of a table or layer, and also disabling the right mouse button ability to save a file. The latter is especially annoying to Windows users who will find all their other right button options inaccessible too. Neither method will deter any but a casual user who isn't trying very hard.

Likewise, embedding your photos in Flash presentations or Acrobat PDFs offer little protection. Tools to capture and extract from these documents are freely available on the web.

And anyway, if it can be seen it can be screen captured. Visible watermarks are currently probably the most effective deterrent, if you find their appearance acceptable.