Don't put them on the Internet. Other than that, everything can be "ripped" if someone really wants it.
Watermarks for one. There are some javascripts but they are annoying. I have a trick I use but only use it on very specific visuals and someone determined can figure out a way around it. I have noticed that some visuals can't be swiped easily. I wanted one for a layout. I wasn't going to use it but wanted it as a placeholder. I could not download it. It kept getting saved as some weird file. I wish I knew how they did it.
If you create graphics that have text that will be hard to remove then typically many won't bother because they won't want other peoples text in it. Here's some examples of bannerse that would be tough to copy. I know some people can do it, but the people that skilled with photoshop generally make their own: This one is just too 3D to really change the text without it looking paste on. With the transparent text in place the poor sucker will have to paste and copy ripples on the ocean to cover this.
you can't physically protect them in anyway so don't even attempt it watermarks help deter people legally you can protect by copywriting - but good luck finding anybody who rips your graphics, and the legal expenses incurred etc. all in it's a failing battle I wouldn't even get involved in; the time can be put to better use
And for graphics, you can always take a screenshot if it won't download. Takes all of 1 minute to SS the graphic, open in Photoshop and cut it out, paste it somewhere, or whatever. BTW, many thieves don't really care if the graphic has your text in it. I was watching an episode of Ace of Cakes and some guy in Mexico stole their images, put them on his site and didn't even bother to remove their text and stuff in the images. Take the echo thing image above. A thief wouldn't care, because people wouldn't know where it was from anyway or care at all. And no point in going through the work because one of the few people who would know would be the owner and that owner would know regardless of the text being there or not.
Worry about bigger issues. But if you see one stolen, send a C&D and contact the host using the DMCA. Or register your copywrite and sue them.
The only real way is to watermark the images, as several have already mentioned. If someone wants to copy your images, then generally they are not worth worrying about, because professional internet marketers would do there own.
One way to deter the bulk of graphic thieves is to set your image as a background within a div, and have a transparent gif stretched out as the picture. <div class="css class referencing actual image as background"> <img src="transparent.gif" width="Actual img width" height="ditto"> </div> Code (markup): This won't stop everyone, but neither will a door lock.
You should watermark your images. By the way there are lot of application online that can do watermarking but the one with lot of options and that allows batch watermaking is at watermark-images.com