That's not appropriate!
Did a random image search in the lab surprise your students with an "unusual" pic? Finding the right picture for an assignment can be problematic. Even with safe search on, you can still get a "range" of images that may not be appropriate for kids. Images are hard to filter because they are not necessarily accurately tagged and can show up even in the most kid friendly search results! Here are a few suggestions:
Infotopia is a custom google search that should search for "safer" images, but it also offers a number of images sites you can search directly as well - take a look at the ones Infotopia suggests. (the buttons below the search window.)
Advertised as a "safe, free image library for education". You'll find copyright-friendly photos and images for classrooms, multimedia projects, web sites, videos, portfolios, or any other project in an educational setting.
"KidRex is a fun and safe search for kids, by kids! KidRex searches emphasize kid-related webpages from across the entire web and are powered by Google Custom Search™ and use Google SafeSearch™ technology."
Remember: no image search is foolproof. Talk to your students about how to handle an inappropriate image or site.