ImageMagick

Homepage: http://imagemagick.org/
Download: ImageMagick

ImageMagick® is a software suite to create, edit, and compose bitmap images. It can read, convert and write images in a variety of formats (about 100) including DPX, EXR, GIF, JPEG, JPEG-2000, PDF, PhotoCD, PNG, Postscript, SVG, and TIFF. Use ImageMagick to translate, flip, mirror, rotate, scale, shear and transform images, adjust image colors, apply various special effects, or draw text, lines, polygons, ellipses and Bezier curves.
Lets see what we can do with ImageMagick:

• Format conversion: convert an image from one format to another (e.g. PNG to JPEG)
• Transform: resize, rotate, crop, flip or trim an image
• Transparency: render portions of an image invisible
• Draw: add shapes or text to an image
• Decorate: add a border or frame to an image
• Special effects: blur, sharpen, threshold, or tint an image
• Image calculator: apply a mathematical expression to an image or image channels
• Text & comments: insert descriptive or artistic text in an image
• Image identification: describe the format and attributes of an image
• Animation: create a GIF animation sequence from a group of images
• Composite: overlap one image over another
• Montage: juxtapose image thumbnails on an image canvas
• High dynamic-range imaging: accurately represent the wide range of intensity levels found in real scenes ranging from the brightest direct sunlight to the deepest darkest shadows
• Large image support: read, process, or write mega- and giga-pixel image sizes

Here you can find some more examples of the real ImageMagick power :D
It is written in C and interfaces to the X library, therefore does not require any other GUI toolkits. ImageMagick is also ported for Windows and Mac OS X.