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You can find an entire gallery of Creative Commons Licensed images on my post : Bitcoin Designs, Free For Everyone To Use with Creative Commons (CC0) License!

A Creative Commons (CC) license is one of several public copyright licenses that enable the free distribution of an otherwise copyrighted work. A CC license is used when an author wants to give people the right to share, use, and build upon a work that they have created.

CC0 “No Rights Reserved”

CC0 enables scientists, educators, artists and other creators and owners of copyright- or database-protected content to waive those interests in their works and thereby place them as completely as possible in the public domain, so that others may freely build upon, enhance and reuse the works for any purposes without restriction under copyright or database law.