old example/javaworld tip that handles how to do this:
http://www.javaworld.com/javaworld/javatips/jw-javatip61.html

I suggest a copyImageToClipboard() method in PApplet. Implementing this
method is as easy as pie, but helpful for everyone using Processing:

BufferedImage clipBoardImage = new BufferedImage(width, height,
(g.format == ARGB) ? BufferedImage.TYPE_INT_ARGB :
BufferedImage.TYPE_INT_RGB);
g.loadPixels();
clipBoardImage.setRGB(0, 0, width, height, g.pixels, 0, width);
ImageSelection.copyImageToClipboard(clipBoardImage);

Code "copied" from PImage.saveImageIO().
ImageSelection class can be found at
http://elliotth.blogspot.com/2005/09/copying-images-to-clipboard-with-java.html
(and works flawless with current JRE).

Thanks, this would be great to add to the 'hacks' section of the site and
much appreciated.

done.
http://processing.org/hacks/hacks:clipboard

Cool, thanks for posting. That may just be the correct place for this
feature request, rather than putting it into the core API. Eventually it
might graduate from hacks and into a library, for instance one that
included options for putting text or other types of data onto the clipboard
(but hides the fully over-abstracted methods provided by the Java APIs).

Also FYI, you can get a BufferedImage by calling getImage() on any PImage
object (which includes PGraphics objects, since they subclass PImage).