Frustration...

So I'm trying to finish my digital story project before I go on my family vacation next week...BUT I'm finding it difficult to find creative commons photographs! I had originally searched for all of my photos on Google Images, but then Dr. Z reminded me about public domain and creative commons. My question is, how did all these OTHER people who had the pictures posted on their sites (so that I was able to find them on Google) gain access to the photos?!
Now, its such a frustrating process to find images that are comparable to the ones I originally pictured in my head! If I can't find a picture of Salvador Dali's "Persistence of Memory" that falls into public domain or that is shared under creative commons, what can I use to take its place in my presentation?! And if I'm having such a tough time, how are my kids supposed to complete the project?!
Reading about the copyright laws has not been any help because they all have so many variations! "The copyright has expired if it falls under circumstances A, B, C, and D, AND even then, only if the author has eaten peanut butter and jelly for lunch more than twice in his or her lifetime." Check it out...it's crazy!
http://www.copyright.cornell.edu/public_domain/