Public Domain images can be used and modified without permission and do not require attribution. For quick projects where you don't want to worry about attribution, these are some great resources.
Flickr.com is a great source for creative commons and openly licensed photographs. To find images, type a term in the search box in the upper right hand corner of the screen. Once you have a set of results, click on the "Any license" drop down menu in the upper left hand corner of the screen. You can choose from various types of licenses, including:
You can verify the license below the image date. To download, click on the downward arrow icon to the lower right when looking at an image. Get links and/or embed a photo by clicking on the arrow icon next to the download button.
Finding openly licensed images on the web is easy with Google Image searches. Simply start an image search and then click on "Tools" to reveal the "Usage rights" drop down menu where you can select the type of use. However, it's not easy to figure out what the exact license is. You'll need to do some digging to find the license and, if creative commons, the attribution information that you need to include. If you hover over the image, you can see which site it is hosted on. Images from Pixabay and Pexels are public domain (so you can use, modify, etc. without needing to provide attribution) but other sources require more investigating. Many images from Wikipedia are creative commons.