The DevIL Image Library For Ruby

DevIL is a fast and lightweight image library that supports the loading and saving of images in almost any format. It also provides some basic image manipulation functionality.
Features:

Loading and saving of images.
Thumbnailing and scaling.
Rotations, cropping, composites, gamma correction, image sharpening, blurring, etc.
Interoperability with the Gosu game development framework.

Example Code:

Devil.with_image("horse.png") do |img|
img.thumbnail(150)
[...]