Pixabay is a good place to find and download quality public domain images. You can search on Pixabay by using keywords or you can simply browse through the library of images. When you find an image you can download it in the size that suits your needs. Registered users do not have to enter a captcha code to download images. Users who do not register can download images, but they do have to enter a captcha code before downloading each picture. 

Applications for Education
Whenever I lead a workshop on video production in the classroom I
recommend that students use their own images whenever possible. When
using their own images is not an option then I recommend using public
domain images. Pixabay is a good place to find public domain images that students can use in their multimedia presentations.


H/T to Dianne Krause.