Developer's Image Library (DevIL) is a cross-platform image library utilizing a simple syntax to load, save, convert, manipulate, filter, and display a variety of images with ease. It is highly portable and has been ported to several platforms.
Use JasPer for .jp2 (and some .icns) support
Use Little CMS for color profiles
Use Libjpeg for .jpg (and some .blp) support
Use Libpng for .png (and some .ico)
Use Libtiff for .tif support