fish.eps

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picture.R

## Draw a fish on a bunch of points, at a bunch of sizes.
grImport::PostScriptTrace("fish.eps")
fish <- grImport::readPicture("fish.eps.xml")

set.seed(1)
x <- runif(10)
y <- runif(10)
lim <- c(-0.2, 1.2)
plot(x, y, xlim=lim, ylim=lim)

vps <- gridBase::baseViewports()
grid::pushViewport(vps$inner, vps$figure, vps$plot)

w <- runif(10, 0, 0.3)
r <- diff(fish@summary@yscale) / diff(fish@summary@xscale)

for (i in seq_along(x)) {
  grImport::grid.picture(fish,
                         grid::unit(x[[i]], "native"),
                         grid::unit(y[[i]], "native"),
                         width=w[[i]], height=w[[i]] * r)
}
grid::popViewport(3)

Awesome, thanks :-)
It looks like you do not need to bother specifying r (line 15) and height (line 21): the aspect ratio of the picture is preserved if you only specify width (at least on my machine).