How to remove background of images with the magick package?

Is it possible to remove the background of images with the magick package?

I know how to use edge detection with Gimp/Inkscape to crop out silhouettes; however, I'm looking to automate the process for a large batch of images with R.

My ultimate goal is to use the ggimage package to plot these images as x,y coordinates but the background of these images is currently overlapping the plot (the dog compared to fink)

library("ggplot2")
library("ggimage")

set.seed(2017-02-21)
d <- data.frame(x = rnorm(10),
                y = rnorm(10),
                image = sample(c("http://www.supercoloring.com/sites/default/files/silhouettes/2015/05/cairn-terrier-black-silhouette.svg", "https://jeroenooms.github.io/images/frink.png"),
                               size=10, replace = TRUE)
                )

ggplot(d, aes(x, y)) + geom_image(aes(image=image))

One can trim the edges of an image using image magick's image_trim()

img <- image_read_svg("http://www.supercoloring.com/sites/default/files/silhouettes/2015/05/cairn-terrier-black-silhouette.svg")
image_trim(img)

but this isn't exactly what I would like.

Any ideas?

The image_transparent() function does this, e.g.:

logo <- image_read("logo:")
image_transparent(logo, 'white')

The white parts of the image will be made transparent, which should be enough for the simple image in your example.