a) The first rule. If you do not want anyone to use your photos, do not publish them.
Once they are published they can be "pirated" in some way or another.
b) Be very clear on the licence of your photos.
Probably you can licence some under a creative commons one, or on the contrary, state that you can not use them in anyway without a fee or a written permision.
There is a large scale of licences you can use.
c) Watermark them.
You can use an intrusive big watermark or a small one.
For example, on this wallpapers I relased under a CC licence:
I added a very small non intrusive credit. Thinking that people would not mind leaving it there.
II. Put a frendly reminder to comment on the usage of your photos.
In some free images sites, the licence for the usage is that you need to comment on where are you using a photo, inclusive asking for a capture of the finished project. This can be interesting on tracking usage, including not only web, but also a printed brochure, etc.
You can also ask for a link to your website. This can be usefull in seo, and after it can be tracked by your host provider.
III. Reverse image search
As already comented by Daniel Chambers, this can be usefull on web published images.