There are a number of different image sites that provide royalty free images. Royalty free means that an image can be used without the requirement to pay ongoing royalties. Some sites like Shutterstock and Adobe stock sell royalty free images but there are a number of websites where you can access free royalty free images. Depending on the terms of the website these images can often be used without attribution.
Images in the public domain are images that do not have copyright including where copyright has expired. These images can be used for free.
When you use an image, particularly an image used under a license such as Creative Commons, you should acknowledge where the image came from and any license conditions associated with the image.
All 6 Creative Commons licenses require attribution to the creator when using or sharing the image. When attributing CC it is best practice to include these four elements.
For more information on how to attribute CC licenced works see the CC Wiki page linked below.
There are a few different search engines that allow you to filter images by the Creative Commons licence. Make sure when using these tools, in particular Google image search, you go to the actual page of the image and confirm the license terms.