And specify your terms of use. Especially on free images. Web sets, design templates for web pages. The only clear way to protect yourself is to register your copyright so you can later go to court with anyone who uses your work without your permission and gain a monetary award from it. We are a long way from perfecting the web regarding copyright issues just as we are in real life.
Screen shots can steal images from websites.
Written words and work is especially hard to protect. Just let them know they are violating a copyright if they do take the images or webpages. Used under License.