You also have an approach to image processing based on "standard" scientific modules: SciPy has a whole package dedicated to image processing: scipy.ndimage. Scipy is in effect the standard general numerical calculations package; it is based on the de facto standard array-manipulation module NumPy: images can also be manipulated as array of numbers. As for image display, Matplotlib (also part of the "science trilogy") makes displaying images quite simple.
Depending on what you mean by "image processing", a better choice might be in the numpy based libraries: mahotas, scikits.image, or scipy.ndimage. All of these work based on numpy arrays, so you can mix and match functions from one library and another.
I started the website http://pythonvision.org which has more information on these.
Introduction to Image Processing with SciPy and NumPy