Another reason they are popular is because they do not have to be the same. A dolphin tattoo can be in about any color you can imagine, and they can be in any size, from a small dolphin tattoo, to one that covers your whole back. I have also seen some amazing dolphin sleeve tattoos that go from your shoulder, to your wrist, and dolphin tribal tattoos are beginning to show up. Some tattoos have the dolphins in their natural environment, and some even have them doing things humans would do.
The best place to find dolphin tattoo designs is an online tattoo gallery. Most web searches will show the same low resolution images many times, but an online tattoo gallery will show high resolution photos, that are printable. This will let you see how your tattoo looks on a person. An online tattoo gallery will have thousands of pictures of dolphin tattoos for you to see, so that you can find a good dolphin tattoo design, and make it one of a kind.