Please note, none of these methods are foolproof, only strong deterrents. Someone with enough computer/internet savvy could get into the source code and track down the original image (without the watermark) on your server. So, if you don’t want any way for your images to be stolen, you probably will want to watermark them before uploading, or not post them at all. This protection though does prevent a vast majority of your visitors from being able to access your images.
