[Bf-cycles] where is cycles image stored?

[Jordan Miller]

I'm rendering a rather large number of cycles (20000 for a complex
scene at very high resolution [2400x3500 pixels]). but part way
through render (after 20 hrs or so) Blender 2.63a crashes and I lose
everything. Where on the HD can I find the image that is continually
being written to? alternatively can I have cycles save a file after
every cycle? it is taking about a minute to render each cycle pass.

please help!

thanks,
jordan