As others have said, images found on Google image search are not free to use unless they are licensed as such. Google does however have advanced search features that will show you images that are licensed for reuse, commercially or non-commercially.
When you are doing an image search, click on "Search tools" then "Usage rights" and you will get a dropdown to filter your search results:
You can read more about it here: Google Search Help: Find free-to-use images
Be sure to check from the source, that any images you find are licensed for free use. Don't blindly trust the search filter, it is by no means perfect.
Better still, find free stock images from one of the many good stock image sites listed here:
Or here:
