Copyright in the U.S.A.
If you put an image, whether it is a work of art, or photograph, on the internet, it is pretty much available to anyone that wants to download and use it! Even if you take steps to protect the image(s). In some cases protection schemes will "spur-on" the pirates to beat your protection widget.
However, there are steps you can take to legally protect your self! The most important method is to file with the US Copyright Office.
You can now file for your copyright on line! Or you can send a cd-rom of the images. If you opt for sending the CD, the fee is slightly more ($45.00). In some cases you may have to send in a cd-rom anyway, especially if the Library of Congress requires a hard copy of your material.
Basically you fill in the form, pay the electronic fee of $35.00 and upload the images (the images don't have to be full resolution, just large enough, in color, and recognizable! You can even .jpg the images and compress them)
Here's an article from Nikon's web site... the links are outdated, but the article contains good information on this process.
Here the link to the United States Copyright Office
I found a real good article at Lucid Images, A Wild Life Photography web site about protecting your images on-line, they even provide free java scripts to aid you with this process.
Invisible Watermarks
Digimark is a company that SELLS software that embeds an invisible digital watermark. But, it ain't cheap.
Using Photoshop Elements for Water Marking Photos
To batch "watermark" many photos in one step in Photoshop Elements use the Menu
File > Process Multiple Files
Then toggle Labels in the right column and in the drop down box choose Watermark...as shown here
A Photoshop911 Watermark action
If you use photoshop here is a good on-line tutorial to make a "watermark action"
If you have any questions about watermarking and copyright issues