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silhoutteAC - More than 47,000 vector silhouettes for free

Openverse - Openverse is an Openly Licensed Images and Audio database and a growing community of photographers, illustrators, and musicians who share their work under a Creative Commons license.

IconBros - Free and continuously growing high quality icons

Pixabay - Over 270,000 free photos, vectors and art illustrations

MakeAppIcon - MakeAppIcon resizes and optimizes your icon designs into all formats needed for iOS and Android mobile app!