How can prevent someone from copying images from a CD by adding a message across image?

I'm not sure how a message could be pasted on top of the picture, but there are other ways you can use to protect others from making hard copies of your photographs. The most common one, is to supply them with a low resolution version of the file.

There are a couple of things you can do, but the bottom line is that is it very difficult to prevent someone from actually getting a copy.

As Aaron2 says, a low resolution image is the safest bet, because then even if they copy it, they can't do much in the form of print or reutilize it for any purpose other than 'screen' display.

No matter what means you apply, unless they are visible on the photo at all times, someone could just do a low resolution 'print screen' capture, where they would keep what's displayed on their monitor on the clipboard, and then can be pasted onto a file, without really physically copying the original. The only thing you can do to 'remedy' this is to have a visible watermark on the image such as seen in the example below:

The real question is what is the nature of the images. Is it that you don't want others to  copy them, or are they confidential and private in nature. If that's the case, I would only display them on a computer where people cannot insert any removable media (jump drives, floppy disks, media cards, etc.) and no internet access so someone cannot take screen shot or a file and attach to an e-mail.

If you're looking to append a visible watermark, you could just open the image in a program like Adobe Photoshop and add a faded layer with text over the image. While it gets in the way of the actual picture, its your surest bet for people not reusing it. For a good tutorial on how to append a visible watermark, visit: http://rs.bahneman.com/protecting_your_images.php or search the web for 'protecting images'.

Good luck! Itay. 

I think you are referring to a watermark. This will not prevent someone from copying your image but if the image will pop up in some web site you will be able to verify that its indeed that image.

If you put a different watermark on images on different CDs you'll even be able to know that this specific picture was copied from a specific CD.

Photoshop as well as GIMP can generate (or help you generate) watermarks which can also include conclease information (like the person to which you gave the image to).