First Some Definitions...
Public Domain = Copyright free, i.e. images that would once have belonged to the creator/copyright holder but are now old enough that copyright no longer applies, or the creator has chosen to make copyright free.
Creative Commons = Creative Commons Licenses - a set of easy to understand copyright licenses that allow people to share work (such as images) in a way that makes it clear whether you can re-use the work.
An Example of a Creative Commons License:
This work is licensed under a Creative Commons Attribution 4.0 International License.
If you saw the above license attached to an image then you would know that you can share and adapt the image in, any way as long as you attribute (cite/reference) the original creator.