Originally posted by DAN513
Or the person could just browse their internet cache and copy the pic from there.

right on bro'
All you have to do is hold the mouse over the image and the file name displays in the status bar at the bottom of the window.
If you see it on a web page then a copy of the image is in your Temporary Internet Files folder.
You can use Java to prevent the image from being copied.
Here is an example:
http://www.webmarketingnow.com/tips/javascript-image-copyright-protection.html
But, to really secure the image as yours you should add a digital watermark so that if it gets reused you can prove it was your image. But as others mentioned they cost $$.
AlphaTec
http://www.alphatecltd.com
Digimarc
http://www.digimarc.com
Signum
http://www.signumtech.com
AlpVision
http://www.alpvision.com
If you want to catch people, SysCop searches the web for your marked images in use.
http://www.mediasec.com/products/index.html
Screen captures are tough to beat. I would agree that video is the best deterent, temporary video streams even better. There are also watermarks being developed called adaptive digital waterwarks that are buried right into the actual pixels of the image. Sort of like those 3d pics that look like nothing but static until you stare at them cross-eyed for 10 minutes.
I worked with a company that sued someone over images stolen from their site. Frigging attorneys cleaned up and all the webmaster had to do was remove the images. There's not much financial recourse unless someone is selling your images or profiting somehow. It also matters whether it's an illustration, photo, video etc...
And as others mentioned, you should create a logo, trademark that
http://www.uspto.gov and apply the logo to your images, which should then be copyrighted
http://www.loc.gov/copyright/ . You can file the forms
http://www.loc.gov/copyright/forms/ yourself for under $100. This gives you two areas of infringement if it's reused without modification. In which case you will need about $200/hr. for a decent attorney to sue them.
Theft has and probably always will be an unfortunate feature of society. Peer pressure is often the best deterent.