library(ggmap)
library(ggplot2)

d <- data.frame(lat=c(33.79245),
                lon=c(-84.32130))

emory <- get_map("Atlanta,Georgia", zoom=12)

p <- ggmap(emory)
p <- p + geom_point(data=d, aes(x=lon, y=lat),
                    color="red", size=20, alpha=0.5)
p