I read the vignette on cowplot. I think we are getting close to a solution but I don't think this is it yet. In the vignette it looks like it calls up and displays images from a system folder or file via the code :
logo_file <- system.file("extdata", "logo.png", package = "cowplot")
ggdraw(p) +
draw_image(logo_file, x = 1, y = 1, hjust = 1, vjust = 1, width = 0.13, height = 0.2)
It looks like it adds saved data to the plot plus a png file of the cowplot logo. However, at this stage of my Rstudio career I can't see how it would load a pdf or png of my choice.