When downloading images from an open collection consider taking a screenshot of the page and store it in a file for future reference. Include the browser bar at the top along with the section that shows the Creative Commons license. If there's a question about your permission to use the item, you'll have proof of the licensing standard on the date you accessed and downloaded it for use.
The following collections contain images that can be freely used. They are made available under creative commons licenses or they are in the public domain. Several of these resources require you to register for free accounts in order to download and use the resources.