Compfight searches for images (many from Flickr) with a slick filtering system (tags, licensing, safe, etc.).
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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).
Tons of images in the public domain that are searchable. You can pay for premium downloads.
All the clip art here is released to the public domain for a frictionless experience! https://openclipart.org/ “All Clipart are Released into the Public Domain. Each artist at Openclipart releases all rights to the images they share at Openclipart. The reason is so that there is no friction in using and sharing images authors make available…
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…
Clip art, photos, and illustrations that are all released under Creative Commons 0! http://www.pixabay.com/ “All images on Pixabay are released into the Public Domain under Creative Commons CC0. Therefore, the images can be modified and may be used freely for any application – also commercially and in printed format. Attribution in appreciated, but not required.”