Needpix offers a collection of over 1.5 million free photos and illustrations under the Creative Commons Zero (CC0) license. As the site states on its “about” page, “[A]ll photos hosted here are collected only from public domain sources such as: pixabay.com, pond5.com, picjumbo.com, publicdomainpictures.net, and others.”
It’s easy to search the well-tagged site. Also, many images are high resolution. Apart from photos, you’ll find vector illustrations, clipart, paintings and drawings, fractals, as well as partially transparent images suited as layers. Look for interesting source and style images for dreaming here!