As I can remember, I was able to search images or pictures according to license type in Wikimedia Commons several years ago, but now, I cannot find the entry to search them. Therefore, I need to think alternative ways to do it.
Here is how I search images or pictures in public domains. Just enter "keywords Category:Public_domain" in the search box. For example, if you want to search "white house" related photos in public domains, you can enter: "white house Category:Public_domain" like this:
Search Public Domain Images or Photos in Wikimedia Commons
This approach is effective for me. I think most of the listed items are public domain, but I don't and can't guarantee it's 100% accurate.