This is an _extremely_ broad subject. Do you have any specific outcomes or transformations you'd like to perform?
If it's for feature recognition or other similarly math-intense tasks, you may find some good answers already exist at http://dsp.stackexchange.com/ (Digital Signal Processing)
OpenCV is a great library for doing various computer vision tasks.
Also, the Graphics Gems series had a lot of the algorithms needed for image processing. It's getting somewhat advanced in age now, but provided an impressive explanation of many graphics operations when I read them last, 10 or 15 years ago.
Also, you can get some interesting code from the 'demo scene' Pouet.net has a massive list. There are also quite a few for which the full source is available. You can almost guarantee that code found there will be fast - even if not commented or written with flexibility in mind.
Another interesting source is http://www.iquilezles.org/
] (He's the author of ShaderToy and a member of the demo-group RGBa)
Often, you'll get great tutorials if you just search for the operation you're trying to implement - i.e Edge Detection, GreyScale Conversion, YUV->RGB conversion, colour-keying, noise removal, dithering, smooth filter, sharpen filter, etc, etc.
Welcome to the glorious world of image processing and remember, have fun!