Many people forget that using someone else's image can violate copyright law. Here are some websites teachers and students can use to ensure they aren't breaking the law.
When finding an image on Google, it is important to change the usage rights to ensure the image is free to use.
If you plan on changing the image in any way make sure to select one of the 'with modification' options.
This site has nice royalty free images and you can log in with your Google account.
Pics for Learning
Find safe photos to use in class with no login required. You also have the option to save the image to your Drive.
Photos for Class
This site has safe G rated images from the the makes of Storyboard That. There is no login required to download the images.
Check out my page for more websites with creative commons photos. Happy image hunting!