The Verge, in a report by @russellbrandom reports that The world's largest photo service just made its pictures free to use. This is Getty Images which "is betting its business on embeddable photos". Up to now the library stamped each image with a transparent square placard reminding you that you don't own the rights. Now Getty Images will allow you to use the images free, though they will add a footer with the image source. I understand that the vast majority of their photos will be available to embed, this means that of course some will still have to be paid for.
I did a quick search for "college classroom" and selected the following image from the 3,165 results that were displayed:
Think of how this could be used by educators! You can find images on any educational topic, below is an embeddable royalty free image of Trinity College:
Or perhaps you are interested in sport? Below is a royalty free image of Pelé from the 1970 World Cup in Mexico. The Getty Library is worth browsing if at times you are stuck for an image to use, and want to do more than just a simple copy & paste with a Google Images search.