Wrong output size when plotting over an image in R

My goal is to read an image file in either the PNG or JPEG format and plot various data over said image and save it to disk.

I also want the image to take up all available space in the produced plot, no axes or labels or anything. I'm a bit concerned that this might be relevant to my problem.

Code example

Below is my current code that currently only tries to output the same image as you put in. Later I plan on plotting data points corresponding to coordinates over the image. I've used some sample code found here in order to remove the axes and be able to have the image in the background of the plot.

library(jpeg)
library(grid)

img <- readJPEG(system.file("img", "Rlogo.jpg", package="jpeg"),native=TRUE)
jpeg(filename = "Rlogo-2.jpg", width=100,height=76, quality = 100,res=299)

op<-par(mar=rep(0,4))
plot(0:100,type="n", axes="FALSE",ann="FALSE")
lim <- par()
rasterImage(img, lim$usr[1], lim$usr[3], lim$usr[2], lim$usr[4])

dev.off()

Example output

This is an example output of my above code in a comparison with the original image:

The image to the left is the original and the right one is the modified one. As you can see it seems as if the image I read and plot somehow is smaller than the original image and when saved to the original dimensions it appears blurred.

I've been pulling my hair over this one for hours and I don't seem to get anywhere. This is my first attempt to plot data over images and I'm aware of my lack of knowledge about how R represents images and I've mostly been using the basic graphics to do relatively simple plots before.

I'm currently considering doing this in Python instead but I'm afraid that'll come back and bite me when it comes to the actual plotting of the data.

I run R version 3.1.0 on x86_64 running Windows 7.