Better than Google
Images
The very safest is just to take all the photos you use yourself!
Teachers and students get a little more copyright wiggle room than the average joe ... but it's still not leeway to use whatever you want whenever you want. And of course we want to model the very best digital citizenship practices for our students and not set them up for trouble later on. I'd like to see more image credits on student posters--if they can print the image they can print the citation! Properly sourcing images does take some time to get used to ... but then it's not hard after that.
Third grade, your kiddos cited the pictures on their notes. That was a start, at least. In the future we'd want to start adding that citation to the project itself. If you want to check the Dropbox folder where all of the images were housed I do have that available.
Britannica Image Quest
Photos For Class
Shadow Puppet Edu (free app)
Creative Commons
Pics4Learning is another.
Always run a couple test searches first!