The CImg Library is an open-source C++ toolkit for image processing.

It mainly consists in a (big) single header file CImg.h providing a set of 
C++ classes and functions that can be used in your own sources, to load/save, 
manage/process and display generic images. It's actually a very simple 
toolkit for coding image processing stuffs in C++ : Just include the header 
file CImg.h, and you are ready to handle images in your C++ programs.

Note: The archives hosted on http://cimg.eu/files/ do not contain the HTML
documentation.  Please refer to http://cimg.eu/reference/index.html or
download a copy from https://github.com/dtschump/CImg