Once a photo is hosted on the web, there is no guaranteed way to prevent people from stealing the photo. You can, though, make it harder for people to copy it. One way to do this is add a plugin to your site that prevents “Right Clicking” — this will make it so that people can’t easily save your photos to their computers.
To do this, click on Add New under the Plugins tab and do a search for “No Right Click” and install and activate one of the plugins you find there.