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 (over 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 Bézier curves.

它不是library, 而是現成的application, 安裝後,
可在php透過command line的system call 來執行它, 如下:

 convert  -negate  image.gif   output.gif

棋安說效能比GD lib好..(但他也沒用過GD, 所以可信度三成... XD)
http://www.imagemagick.org/script/index.php