Hello
First of all you do not know they are "stealing" you pictures. If you put your photos online then anyone visiting your pages will have downloaded the pictures to their computer. Any time you visit a page on the internet the entire content of that page is downloaded to your computer. All you might know is that someone accessed a photo directly, which still doesn't mean they are stealing it, they could have just clicked a direct link to it.
Now, if someone is using your photos without your consent then that is another issue, but just because they have accessed them is no indication that they are doing so.
There is no way to protect your images and have them available online. There are "hacks" that make it only moderately more difficult to "copy" your images without having to go into the browser cache. (But, they are still in the cache to get anyway). You could diable right-clicking, put a transparent gif over the image so the user can't right-click and save as, etc. But, then there is always the Print Screen button.
My advice is to put a watermark on your photos. That way it is very difficult for anyone to repurpose your photos without others knowing they took them from your site.