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 Bézier curves.
The functionality of ImageMagick is typically utilized from the command line or you can use the features from programs written in your favorite programming language.
One thing I found that I like, and which seems obvious but I never thought of before is using query strings in image source attributes to dynamically create images. Like: <img src="image.jpg?query=Some Text For Image">
Also been looking for some examples of web-based graphics editors (preferably open source) (I want to build one). Found an open source one by Pete Frueh. It uses the GD library. Here's a demo of it. Here's the article where I found out about it: Open source PHP-based Ajax image editor.
And I came across a comparison from Anthony Thyssen's writing, which I found really interesting: Comparison of Povray and OpenGL.