my class about classroom blogs we had a short discussion about copyright, Creative Commons, and public domain media. As I always do, I made the recommendation that students should always try to use their own media before looking for public domain and or Creative Commons licensed media. Before starting any blogging activity or other multimedia presentation assignment, I think it is good to review with students the basics of copyright and Creative Commons. The following two videos from Common Craft provide excellent overviews of these topics.
For a more in-depth look at copyright for educators, watch Dr. Wesley Fryer's Slideshare on the topic. Six years after he released it, it's still one of the best resources on the topic.
Common Craft videos can be viewed for free online but to download them or embed them you do have to be a subscriber to their service. Disclosure: I have an in-kind business relationship with Common Craft.