The online world is made of bits, and the very nature of the web makes it infinitely easier to copy something than to prevent it from being copied. Whether it's an image, a video, a song or text, it's all 1s and 0s, and all it takes is a download option to make a copy of anything you want.
The web was started as a publishing platform, where the owner of a server would put up content, usually created by them, and then users would consume it. But only a few years passed before this all changed. Now, there's social network sites, photo sharing sites, blogs, and so on. Downloading an image, and uploading it again, takes no time at all, and it doesn't take long to find truck loads of image galleries that were obviously not created by the one who uploaded it.
The other issue is how blurry the moral and legal implications are. Many people don't realize what is and isn't legit online, and with the added anonymity, that makes it hard if not impossible to track down whoever is at fault.
Why do it?
Why would someone create a Facebook account, then upload thousands of pictures of celebrities that they don't own? Why would someone share on their Tumblr account scanned pages of comic books or magazines? The reason is usually pretty innocent and simple. They like the images, and want to share them with their friends. It's rarely about stealing, it's more often about wanting to enjoy the content.
For most people however, who spent long hours creating something, they want a reasonable solution. One popular option is watermarks. It's easy enough to put a signature on an image, linking to your own site, and if people share it then you actually gain exposure by it. That's actually what a lot of artists do, and they get traffic from having their work plastered around the web.
Finally there's the harsher way, the one companies have been using, which is demand the copies be taken down. As the music and movie industry have recently learned however, that doesn't really work very well. Sending DMCA notices, or suing for non compliance, is a long and costly process, whereas setting up a new site to upload the same work of art takes no time at all.
I think in the future, we'll see a combination of various approaches work best. Using the online sharing phenomenon as marketing will prove to be a powerful way to get your name known. Right now we're seeing the first stage with things like Facebook like
buttons and other sharing features that anyone can implement on their sites.
As services become more integrated however, that will soon be even more powerful. Think about a future where all images include metadata that is integrated by sharing sides. When you upload an image, the site knows right away where it came from, and automatically credits the author.
Most users will go for whatever is easiest. As long as the solution is faster than clicking on download, and then sharing it back up, it will be adopted by users.