Anyone who has been given access to your Facebook photos is able to click the "Download" link beneath an image, and save it to her computer. If you do not want strangers to download your images, just restrict your photo album privacy to "Friends Only." If there is a particular person on your friend list that you don't want to steal your photos, block her from viewing the album that contains the photos. People are never notified when you restrict them from seeing content on your profile, including photos.
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Go to the "Account" link at the top of Facebook, and select "Privacy Settings" from the menu.
Click the blue "Customize settings" link.
Scroll to the bottom of the "Things I Share" section, and click on the "Edit privacy settings for existing photo albums and videos" link.
Click on the gray icon beneath an album, and choose "Customize." Enter a person's name in the "Hide this from these people" field, then click the friend's name when Facebook shows it, along with a copy of her thumbnail image. Repeat the process for each album you want to block the person from.