Flickr can be a good place to find Creative Commons-licensed images to use in multimedia projects. That said, Flickr does have its flaws and some inappropriate pictures can slip through its filters. One way to avoid this problem is to build your own Google Custom Search Engine. In your search engine you can specify the image hosts that you want your search engine to index. When your students use your search engine the only content that will appear will be from the sites that you have deemed appropriate for them to view. You could start building your search engine with this list of sites that host public domain and Creative Commons-licensed content.
How to create a Google Custom Search Engine.
Important note for building an image search engine: on the page in which you specify the sites to index you must enable Image Search.