Free downloadable historic maps of Chester County can be found at the Library of Congress, and at the New York Public Library among its state map collection (scroll down to find PA or a subcategory under PA). The Pennsylvania Historical and Museum Commission (PHMC) also has an online map collection. The University of Texas Libraries legacy website has maps that are mostly freely downloadable in the Perry-Castañeda Library Map Collection, with many historic maps of the United States (please read the Maps FAQ).
Historic aerial photographs of Pennsylvania are available at Penn Pilot Photo Centers. You will need to specify a location, such as "Franklin Township, Chester County, PA" and can select from from three time periods. Each red dot on the resulting map represents a viewable and downloadable aerial photograph.
Historic Map Works has a wide range of maps for sale, but watermarked maps are also free to view. They have the 1873 Witmer atlas of Chester County and the 1883 Breou's atlas of the county. Or you can browse the entire collection. Another commercial source for images of PA and Chester County is Andy's Antique Atlases. The images are viewable as watermarked PDF files.
Other useful websites are those of Historical Maps of Pennsylvania and the Greater Philadelphia GeoHistory Network. The David Rumsey Map Collection is extensive and freely viewable, with a good collection of Pennsylvania maps.