tik.R

library(tikzDevice)
library(ggplot2)
options(tikzLatexPackages =
          c(getOption('tikzLatexPackages'),"\\usepackage{graphics}\n"))

d = data.frame(x=1:10, y=1:10, f=factor(sample(letters[1:2], 10, repl=TRUE))) 

p <- qplot(x,y,data=d) + theme_bw() + 
  theme(plot.margin = unit(c(1, 1, 5, 1), "lines"), 
       axis.text.x = element_text(size = 12 * 
                                  0.8, lineheight = 0.9, vjust = 10)) + 
  scale_x_continuous(breaks = c(2, 8), labels=c("\\includegraphics{a.png}", "\\includegraphics{b.png}")) 

tikz("annotation.tex",standAlone=T,width=4,height=4) 
print(p) 
dev.off() 

system('pdflatex annotation.tex && open annotation.pdf')