I am trying to find a source of spatially referenced aerial photos that can be analyzed with ArcGIS or R. I cannot use Google Earth because I have nothing to georeference it to. I know they are out there but it has been years since I have used to program. Specifically, I am looking for Napa County, California.
USGS' Earth Explorer is an excellent resource for this type of imagery (unless the US Govt is on shutdown).
In this case I would look for NAIP imagery in your area of interest.