The site plan image is quite straight forward. You import an image into Sketchup.
Draw a base the size of the image you want to import. Import image and it paints as a material on the surface. Adjust scale. I played around with this a bit. I believe that png files work as well. You can also import an ACAD dwg file. With ACAD, be sure to put ALL content on layer 0 if you do not want all the associated ACAD layers coming into SKP. I add the base ACAD image to all of my models then hide it when I want. If you IRender the image, base sheet turns out very crisp.