Processing is a open source graphical library. Simply download the image that has processing installed. This includes examples and libraries. If you are new to Raspberry Pi you can Get Started using the Raspberry Pi foundation guides.
Once you have written the image to the SD card you can start the image in the Raspberry Pi and you can start Processing from the Programming Menu. The Web site includes a nice snippet on setting up processing:
The sky's the limit if you want to be creative and express your inner artist. I started with running example sketches. The next step will be uploading my images to examples.
