How to generate graphics and write the image directly to a file
Have a look at the GraphicsAndImages page. It depends somewhat on the kind of graphics you want to produce: some libraries like matplotlib are focused on graphs and charts, whereas the PythonImagingLibrary supports both bitmap manipulation as well as line and shape drawing (and other similar operations) using the ImageDraw module.
Many on-screen graphics packages also support drawing onto and the saving of bitmaps (eg. PyGame), and it is possible to draw to off-screen bitmaps or devices in various graphical user interface toolkits and frameworks - see GuiProgramming for details of the different choices.
An example of printing some off-screen picture using PyQt (rather than writing a file, although the principles are the same) can be found in the following message: "Placing graphics & text on printed page - jan06call.jpg (0/1)". -- PaulBoddie