Better than Google
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
This one is the most vetted of any source ... the least likely to come up with "strange" images. They also include citations in a text file. Citations are one thing it's OK to cut and paste! Actually all sites on the library resources page will offer the citations for images.
Photos For Class
This one automatically cites any picture you choose ... when you download the pic it will already have the source attached at the bottom. It's fairly safe but not guaranteed so run a couple test searches on keywords kids might be using before you turn them loose.
Shadow Puppet Edu (free app)
This is a good place if the kids are just looking for images to use in a slide presentation. It's as close to a replacement for Haiku Deck as I've found. It will automatically give the photos credit at the end of the presentation. Again, best to run a couple test searches on keywords first.