Photosforclass.com includes built-in #creativecommons search engine. Great search engine for free images that include image citations. Safe G Rated images.
This a search engine for free photos. These come from many sources and are license-specific. You can view a photo's license by clicking on the license icon, below and left of photos. Membership is free and allows you to rate, tag, collect and comment on photos.
Fantastic resource that not only provides resources for teachers and students, but allows for CURATION & customized collections. This could be a great way to have students learn CURATION skills - an important digital literacy. Google Apps integration.
Giphy (a search engine for GIFs), created a code that makes GIFs actually loop on Facebook & Twitter. It works simply like posting a link to a video. Either copy paste the URL of the page for the GIF you've selected, or embed straight to Facebook & Twitter from that page from the portion where it says "share GIF.