Tons of modern CC-BY icons! Search by keyword, and then choose the file type (.png, .psd, .svg, .eps) and the size. An incredible collection that will cover most of your icon needs.
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All the images at Wikimedia Commons are either public domain or licensed under Creative Commons (the specific licenses vary). There’s a good search mechanism and the attribution is easily accessible
A wonderful directory of public domain images and illustrations. There is a good search function, and even web-based tools for creating and editing SVG files.
A great collection of vector art – and you can modify it online before downloading!. Some art is not CC, but the majority of it is. Downloads include .jpg, .ai, .pdf, and .eps).
A great site with resources for designers (icons, photos, stock images, etc.). There is a freemium model, and you can pay for access to more content, but there are thousands of open assets here. http://www.freepik.com/ “Freepik offers users, high quality graphic designs: exclusive illustrations and graphic resources carefully selected by our design team in order…