Available C++ Libraries FAQ
Libraries available to download (D-L)
Image Magick - Image manipulation application and librariesJeff Donner
It isn't C++ but C, but it's so cross-platform and comprehensive an image package, I thought you should know about it. If you look at their page you don't see anything about the libs, but they (and all source) are included with the app. They're free, (commercial use OK, copyright only) and useful.
ImageMagick does a pretty thorough job of reading & writing most image formats.
Added : 1998-12-03 Amended: 2003-03-13 Licensing : Copyrighted freeware
Copyright (c) 2018 Nikki Locke, Trumphurst Ltd.
Permission is granted to distribute over the Internet without charge. The author's permission is required (and usually given) to distribute in any other way, including on CD.