CImg Library — cimg.sourceforge.net
The CImg Library is an open source, C++ toolkit for image processing, designed with these properties in mind.
pHash — phash.org
pHash is an open source software library released under the GPLv3 license that implements several perceptual hashing algorithms, and provides a C-like API to use those functions in your own programs. pHash itself is written in C++.
Implementation and Benchmarking of Perceptual Image Hash Functions — phash.org