This pure Ruby library can read and write PNG images without depending on an external image library, like RMagick. It tries to be memory efficient and reasonably fast. It supports reading and writing all PNG variants that are defined in the specification, with one limitation: only 8-bit color depth is supported. It supports all transparency, interlacing and filtering options the PNG specifications allows. It can also read and write textual metadata from PNG files. Low-level read/write access to PNG chunks is also possible. This library supports simple drawing on the image canvas and simple operations like alpha composition and cropping. Finally, it can import from and export to RMagick for interoperability. Also, have a look at OilyPNG at http://github.com/wvanbergen/oily_png. OilyPNG is a drop in mixin module that implements some of the ChunkyPNG algorithms in C, which provides a massive speed boost to encoding and decoding.
Manipulate images with minimal use of memory via ImageMagick / GraphicsMagick
RMagick is an interface between Ruby and ImageMagick.
paperclip-optimizer is a Paperclip processor for optimizing and minifying uploaded images.
A ruby ImageMagick/GraphicsMagick library that doesn't suck
Ruby bindings for cairo
Thin ImageMagick DSL with smart command composition
Ruby/Pango is a Ruby binding of pango-1.x.
Ruby/RSVG2 is a Ruby binding of librsvg-2.x.
ImageScience is a clean and happy Ruby library that generates thumbnails -- and kicks the living crap out of RMagick. Oh, and it doesn't leak memory like a sieve. :) For more information including build steps, see http://seattlerb.rubyforge.org/
Simple Ruby stateless module interface to imagemagick.
RMagick replacement which uses ImageMagick directly by constructing chainable commands
a library for automated lossless image optimization
Ruby bindings for DevIL cross platform image loading library
Simple image manipulation using Apple's Core Image technology. Scale, rotate, flip, convert, overlay, and more.
A service for image manipulation - built as an extensible wrapper of Imagemagick which handles caching, c/o the Dragonfly gem.