These are all excellent sources to find images that you can legally use on the web. Just searching Google images to find a picture or piece of art, downloading, and loading to your own website violates copyright laws. Be careful to read and understand the license agreements for each image.
Usually, creative commons licenses requires only an attribution. There may be restrictions regarding commercial use or making modifications to the original image.
Usually, royalty free images requires some payment. Some sources may charge hundreds of dollars for images. Other sources provide royalty free images for less than $1.00. Again, make sure you read the license agreements.
Images which fall within the public domain do not require attribution or payment. There are many excellent images. I do tend to find, however, that the royalty free image resources tend to have higher quality settings, design, or color. Neverthless, there are many, many excellent public domain photos. Be sure to read the license agreements.