Personally, I can recommend using PixaBay and Pexels.
They offer "free", "CC0" images. (Creative Commons, 0)
And Wikipedia can be a good source of "CC-BY-SA". (Creative Commons, By Name, Share Alike)
Here, simply state
- Where you found it?
- Who uploaded it?
- Which license has it? (For the record)
Par example:
"Mr. Ding Dong, Wikipedia, CC-BY-SA. [Hyperlink to source]"
"PixaBay.com, CC0 License. [optional Hyperlink to source]"