CImg stands for Cool Image: it is simple to use and efficient.

. The CImg Library is a free C++ toolkit providing simple classes and functions
  to load, save, process and display images in your own C++ code.
. It is highly portable and fully works on Unix/X11, Windows and MacOS X
  operating systems. It should compile on other systems as well (eventually
  without display capabilities).
. It consists only of a single header file CImg.h that must be included in
  your program source.
. It contains useful image processing algorithms for loading/saving, resizing/
  rotating, filtering, object drawing (text, lines, faces, ellipses,..), etc.
. Images are instancied by a class able to represent images up to 4-dimension
  wide (from 1-D scalar signals to 3-D volumes of vector-valued pixels), with
  template pixel types.
. It depends on a minimal number of libraries: you can compile it with only
  standard C libraries. No need for exotic libraries and complex dependencies.
. Additional features appear with the use of GraphicsMagick: install the
  GraphicsMagick package to be able to load and save compressed image formats
  (GIF,BMP,TIF,JPG,PNG,...).
. Additional features appear with the use of LAPACK: link your code with the
  lapack library to be able to compute eigenvalues or eigenvectors of big
  matrices.