How to Search for Images
Google Advance Search
Free to use or share: This setting allows you to use the pictures and share them with others, as long as you don’t modify them, or use them in a commercial application, such as on you website with ads.
Free to use or share, even commerically: This setting will show you picturest that you can use to make money, but you still can’t modify them.
Free to use share or modify: The images you can share with others, and use them in things like reports, and slideshows, and even modify them for these applications, but they can not be used for commerical gain.
Free to use, share or modify, even commercially: These images can be used in any application, even ones that make money, and can even be modified for use in those situations.
In all of these, even when an image is in the Public domain to modify and use commercially, give credit to the original authors and contributors.