image or rgbimage plots with zero decorations

Hi. I'm pretty sure this isn't possible, but I'd like to ask anyway.

I have a gnuplot script to plot an image, with some extra stuff on top
of it. Something like this:

  set terminal png
  set output "out.png"
  plot "image.png" binary filetype=png flipy with rgbimage, "data" with lines

Ideally I would like the result to be a .png file with the same
dimensions as "image.png", and I would like "image.png" to fill the
whole output plot. I.e. I want to annotate "image.png", but not scale it
in any way.

There are some options that help somewhat, but don't do everything:

  set autoscale noextend
  set size ratio -1

In practice, what I end up doing is to manually find the pixel size for
the "set terminal png" command that results in the output image of the
correct size, and then I crop "out.png". Is there a better way?

Thanks.