The Getty Museum makes freely available all of the images to which they hold the rights or in the public domain. Read more about this project. They ask that you use this credit when using a digital image: Digital image courtesy of the Getty's Open Content Program.
The Noun Project is a collection of openly licensed icons. Most icons can be downloaded as both .PNG or .SVG files. All icons can be downloaded free of copyright for a small fee (usually a couple dollars), or free of charge under the terms of a Creative Commons license. Note that when an icon is downloaded free of charge, the illustrator's name is in the lower left corner of the image. Attribution must still be given in an appropriate place near the use of the image.