Starting today, you'll be able to download these and over one million public domain books from Google Books in an additional format. We're excited to now offer downloads in EPUB format, a free and open industry standard for electronic books. It's supported by a wide variety of applications, so once you download a book, you'll be able to read it on any device or through any reading application that supports the format. That means that people will be able to access public domain works that we've digitized from libraries around the world in more ways, including some that haven't even been built or imagined yet.
Library of Congress Photo Archives is a site every teacher should bookmark. With over 1.2 millions images in this database, your students can certainly gather a wide variety of images for their history projects. Each image has different licensing, so look closely. Supporting units: famous Americans, presidents, civil rights, wars, inventors, authors, and just about any historical American event
They left out Flickr Commons, though, which is a really excellent international source and allows for public tagging if you want to curate things with your own search terms: http://www.flickr.com/commons/