For the last couple of years Pixabay has been my go-to resource for public domain images. Pixabay was recently updated to make it easier than ever to find quality public domain images. Now when you visit the search page on Pixabay you can filter your search according to image type (photo, drawing, vector), image orientation (landscape or portrait), and image category (subject matter).
When you find an image on Pixabay you can download it in the size that suits your needs. Most images are offered in at least three sizes. Registered users (registration is free) 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.
One of the aspects of Pixabay that does confuse some first-time users is the display of Shutterstock images at the top of the results page. The Shutterstock images are not free. Everything displayed below the row of Shutterstock images is free.