It Appears FCPX optimises all stills to ProRes 422 at their original resolution. Not sure at what version this started as I skipped from 10.02 to 10.05.
Stumbled upon this when importing the BlackMagic Cinema DNGs. You can read about it
here. Have tested with jpegs, tifs and Canon raw.
Render files are created within the Event folder /Render Files/High Quality Media.
Files are created on import if "create optimised media" is selected or when playing the still in the event, project timeline or project browser. Screenshot below shows the render files created when importing a RAW file from my 60D, dropping it into a ProRes 422 1080p 24 project and rendering. I can't actually view the data in the image files labelled "Frame 0 -1023" but the accompanying plist (show open in Xcode but you can see more info in TextEdit) appears to store the raster size and codec. Also the file size of the 18Mpx optimised 60D still is about what you'd expect if encoded in ProRes422. At 5.1 MB it's around 8.5 times the size of the 2Mpx 1080p render file.
Hope this info helps. As far as I'm aware it's not documented anywhere.