Kid Friendly Picture sites?

Hi

I am starting a project with my students in January where they will be compiling pictures on various topics. I thought it would be a good opportunity to introduce them to the concept of copyright and the process they should think through when they use pictures.

I know alot of teachers seem to use Flickr as a source for students to get pictures. It would be great for this purpose as it has a variety of licenses which would help me explain that part of the project , but in all honesty I see pictures on the site which are not ideal for school use.

What picture sources are teachers using with their students? I am not looking for royalty free, etc-- just a safe place to get some pictures! My students are 9th graders.

Thanks! :)

Sue