STB single-header image loading library. [Public Domain]

Open source library for face detection in images. The face detection speed can reach 1500FPS. [BSD]

The “Quite OK Image Format” for fast, lossless image compression. [MIT]

Improved JPEG encoder. [BSD/BSD-3-Clause/ZLIB]

A JPEG image codec that uses SIMD instructions to accelerate baseline JPEG encoding and decoding. [IJG & BSD-3-Clause & zlib] website

JPEG XL image format reference implementation. [BSD-3-Clause]

PNG encoder and decoder in C and C++. [Zlib]

C++ image processing library with using of SIMD: SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX-512, VMX(Altivec) and VSX(Power7), NEON for ARM. [MIT]

Powerful image and texture wrangling library with support for a wide number of common lossy and RAW formats. [Modified BSD]

Leptonica is an open source library containing software that is broadly useful for image processing and image analysis applications. [BSD-2-Clause] website

Library for encoding and decoding .avif files. [BSD-2-Clause]

the reference library for use in applications that read, create, and manipulate PNG (Portable Network Graphics) raster image files. [libpng-2.0] website

an open-source JPEG 2000 codec written in C language. [BSD-2-Clause]

GD Graphics Library, famously used in PHP for image loading/manipulation & thumbnail generation. [custom permissive license, requires mention in user docs] website

Super fast C++ .PNG writer/reader. [Unlicense]

Simple, modern libpng alternative. [BSD-2] website

A C++14 high performance video and image processing library. [MIT]

A generic C++ computer vision library for image analysis. [MIT X11]

Easy-to-use cross-platform image decoding library with pluggable image codecs. [MIT]

Header-only Tiny DNG/TIFF loader and writer in C++. [MIT]

lightweight TIFF reader/writer library. [GPL-3.0]

Tiny ISO-compliant C++ EXIF and XMP parsing library for JPEG. [MIT]

A fast image processing library with low memory needs. [LGPL] website

Simple and Fast header only Bitmap C++ library. [MIT]

A small, open source, C++ toolkit for image processing. [Own LGPL or GPL]

An image processing and conversion library to load, save, display, transform BMP, JPEG, GIF, PNG, TIFF, MNG, ICO, PCX, TGA, WMF, WBMP, JBG, J2K images. [zlib]

An open source library that supports popular graphics image formats and others as needed by today's multimedia applications. [GPL2 or GPL3]

DICOM Toolkit.

Grassroots DICOM library.

An open-source, cross-platform system for image analysis. [Apache2 from ITK 4.0]

ImageMagick program interfaces for C++. [Apache2]

ImageMagick program interfaces for C. [Apache2]

Cross-platform library for high dynamic range imaging. [Modified BSDF]

An OCR engine. [Apache2]

Open-source, freely available software system for 3D computer graphics, image processing and visualization. [BSD]