Here is a link on how to watermark a photo using GIMP (an open source alternative to PhotoShop)
There are actually many alternatives you might consider as well.
if you are using a CMS (content management system like Wordpress, Joomla et all) you should be able to have watermarks created dynamically so you don't have to manually watermark each image before you upload it. If you're not familiar with programming you could hire a programmer to do this for you.
You can also limit this by blocking certain user agents in your .htaccess file (cURL wget would be some common ones) this isn't a complete fix as you can spoof your user agent fairly easily but it would stop the lazy folks that don't bother.
Some people also disable the ability to right click this would stop humans from right clicking and "save as" your images as opposed to scripts in the previous example.
There are also scripts so that when people do use your images the code they copy automatically inserts a link back to you attributing you are the image creator. (I can't find a link, but if I come across it I'll post it here).
I'm sure there are more options but hopefully this gave you some ideas.