As @Acorn stated, there really is no 100% sure way and making them that small is a waste of your time and effort. Unless you put them behind a paywall and just Show thumbnails, the really only way is legally, that requires you to file Copyright protection documents, and put a statement on the site if it is yours that the images are not to be used "__________________" fill in the blank, but at the same time you have to follow every image and where it's used and how. There is the water mark approach but then that takes away from the image unless you plan selling a none water marked version.
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