So, how do you put images online and have some assurance that someone's not going to steal it? Well, you don't. However, there are things you can do to track images, or at least, be able to "mark" your image in some way that identifies it as yours. But be forewarned, there are many ways to do this, and there are pros and cons to each. Regardless of what the marketing material may say, no method is entirely failsafe.
Essentially, there are two methods: a visual watermark, and a "digital" watermark. The term comes from the days when paper itself had identifying information embedded within it in such a way that it could not be removed. You may see this on the paper currencies from most countries in the world. It is used as a failsafe measure to make sure that counterfeiters don't create their own currency, diluting the value of the original.
The visual watermark on a digital file is rather simple: you can make an image that has your copyright symbol or other identifying visual on the picture itself. If people copy the picture, you can still see it's yours. You can choose to put a huge mark in the middle, or you can put a more subtle mark on the side. While it's easier to see the entire image when using a small mark, the disadvantage is that it can be easily removed by an unscrupulous person using a simple image editor. While this is nearly impossible with a larger mark in the middle of the image, the strong disadvantage is that it's harder to see the image itself.
As a response to this form of image identification, a new developing technology is the "digital watermark", which is a process by which an identifying number (yours) is embedded in an encrypted manner within the pixels of an image so that a counter-part "reader" program can identify the owner of the image. The way this is done is by adjusting certain features of the image, such as hue and saturation, in such a way that its intent is to be inpercetible.
There are many technologies and producs out there that do this, but the leader in this area is Digimarc (www.digimarc.com). Regardless of the implementation of this technology, it works by having the user register themselves with the service, and he is returned a unique ID in the form of a number. The user then uses a "writer" program and a "reader" program that embeds or reads this unique number from an image. Once an image has been embedded with an ID, the technology provider offers a service where they scour the internet, searching for images that have their digital signature. You get a report on where they've found your images. The fee they charge for this varies depending on the number of images you want them to keep track of.
The intent is for the image to not have any visual degradation like that of the visual watermark method, and to make it impossible to remove the mark. If accomplished, the digital watermark would be an excellent approach to digitally locking and tracking your images. However, the problem that all of these technologies have is that, despite their claims to the contrary, they "soften" the image. It varies from image to image, but most of the time, sharpness is dramatically reduced, especially in more "complex" images with high contrast. This is personal taste and ultimately a business decision on whether you want to do this. I choose not to because I need the sharpness of my images. You can compare if you like:
I used to digimarc all the images on my site, but I actually had too many people tell me that the images just weren't appealing enough because of the fuzziness. (Almost all the images on my site have been rescanned and uploaded without the digimarc -- the Ecuador images are one of the few remaining holdouts.)
Of course, this is a personal decision, and you should decide for yourself and compare your own images with and without this feature.
There is another alternative as well: embed your copyright information as plain text in the image file itself. (You can do this using photoshop and most other image editing programs.)
Given all this, recognize that there is a practical and pragmatic reality to all this: 90% of the time, people who "use" your images don't edit out the visual mark. Of the remaining 10% that do, 90% of them won't think to edit the .jpg file itself to remove the embedded copyright text. You actually have to edit the image to take it out nearly the same way it took to put it in. Sure, it CAN be done, and is therefore not completely safe, and that's the basis for arguments in favor of digital watermarking: it's more difficult to remove a digital mark.
Remember, watermarking your images -- whether visually, digitall, or by embedding text in the image file -- doesn't prevent people from stealing images. It just makes it easier to track your image on the net. This is why you have to make a bigger decision: do you want to have images on the net at all? And if so, what size?
In the end, if you choose to digitally watermark your images, you have to decide which of the two evils you're willing to accept:
1) safe digital
mark at the expense of visual degradation
I chose 2 because I work under the following philosophy:
Don't put so much work into preventing people from stealing your images as much as the work you do to helping people buy your images. Anyone willing to pay big bucks for an image isn't the type of company that'd steal it. If someone is going to steal it, chances are they weren't about to pay much for it in the first place.
In summary, I'd rather get 90% of a much bigger pie than 100% of a very small one.
For an in-depth essay on this subject, see: http://www.danheller.com/ep-webuse.