The images are real homeland security symbols. In all seriousness, my friend Chris Mayhorn from North Carolina State University has researched these homeland security symbols and found that not everyone can interpret the real meaning of these warnings:
Mayhorn, C. B., Wogalter, M. S., & Bell, J. L. (2004). Are we ready? Misunderstanding homeland security safety symbols. Ergonomics in Design, 12, 6-14.