The simple answer to this is also the most hated. You can’t.
Artists, Photographers, News/Stock Agencies have been asking this question forever and there is no simple solution.Because of how easy and inexpensive it is to replicate something digitally, If you put up content it will be stolen.
Here are the methods currently used and ways that people can circumvent them.
The Infamous right-click disabled.
Just because they cant right click and save doesn’t mean they can’t download the image.
Users can take a screenshot [Cmd + Shift + 3, on Mac, PrintScreen key in Windows]
They can get the image out of the browser cache, which is saved on their hard-disks.
They can also view your source code and find the link to the image file.
Transparent Gif Overlay
Beaten just as easily by screenshots, browser cache and viewing the source.
The unobtrusive watermark
Just because the image has a watermark in a corner doesn’t mean it wont be taken and used.
People will crop the image or photoshop over the watermark and use the image without permission anyway.
The obtrusive watermark
This is the only method that can deter copyright violation but it also degrades your art.
Sometimes instead of helping it can hurt your photos, by distracting potential buyers from the image itself and focusing on the watermark.
This is often used in Stock Photography websites where such protection is a requirement.
Prevention is better than cure
There are actionable steps you can take to prevent your images from being copied.
1) Dont display full-size images.
Never upload images that are large enough to be used in commercial work. Keep your images under 600px wide. This way if they are blown up in an image editor the image will pixelate and become unusable for any serious reproduction.
2) Keep your images out of search engines.
Don’t let google index your images directory. This will work for other search engines also. Keep all your images in an images directory and block access to it via your robots.txt file. This way you can keep your art out of ‘image search’ results.
3) Let people know the images are protected.
Display your copyright and usage policy alongside the images. Have a clear way for people to contact you for permission to reproduce.Use Creative-Commons to let people know what they can and cannot do with your images
4) Use a Digital Watermarking Service like DigiMarc.
Although these services cannot stop people from copying your content, they will help identify copyright infringement and allow you to contact offenders.
If they can see it, they can steal it. The only way to guarantee that your images wont be stolen is to never upload them to the web in the first place.