The U.S. Fish and Wildlife Service has hundreds of environment-related images, videos and audio clips that are free for the public to use without permission. The pictures include birds, plants, environmental conservation, fisheries, habitats, landscapes, fisheries, invertebrates, wildlife, and historic images. Videos include several about habitats, including schoolyard habitats.
Open Clip Art Library has a large collection of more than 40,000 images free for anyone to use. There are tons of images here useful to teachers and students. Some of the clipart is in collection packages on themes such as spring, sports, Thanksgiving, etc. Or you can search for individual images. There is lots of science clipart for astronomy, plants, chemistry, and physics. There are images of kids and animals and clip art for math concepts, historic figures, cultures and countries, to name a few. You have the ability to use an online editor for many of the images.
WP Clipart has a collection of more than 77,000 images free for teachers and students. File formats vary by image and consist of JPG, PNG, SVG, and WEBP. You have the ability to do some simple image edits on the site, such as resizing, rotating, cropping, and changing brightness and contrast. The site has minimal ads and no offensive images, so is suitable for children to browse.
Wikimedia Commons is a huge media file repository, making available public domain and freely-licensed educational media content (images, sound and video clips) to everyone. There are a broad range of file types. You have the ability to hotlink an image or video to your classroom website. Check the attribution requirements that are specific to piece of media.
Pics4Learning is a copyright-friendly, curated image library for teachers and students. The collection consists of thousands of images that have been donated by students, teachers, and amateur photographers. Unlike many Internet sites, permission has been granted for teachers and students to use all of the images donated to the Pics4Learning collection.