Can geom_image() from the ggimage package be made to preserve the image aspect ratio?

When researching this answer, I tried to draw the image strip via geom_image() from the ggimage package but couldn't get it to work. geom_image() modifies the images aspect ratios, and I don't know how to prevent it from doing it (or whether that is even possible). It's also not clear to me in what units size is measured. From how the code behaves, maybe it's in npc coordinates running from 0 to 1, regardless of the ggplot2 coordinate system?

Here is the code I used:

require(ggimage)
df_img <- data.frame(phase = c("Interphase", "Prophase", "Metaphase", "Anaphase", "Telophase"),
                     image = c("http://www.microbehunter.com/wp/wp-content/uploads/2009/lily_interphase.jpg",
                               "http://www.microbehunter.com/wp/wp-content/uploads/2009/lily_prophase.jpg",
                               "http://www.microbehunter.com/wp/wp-content/uploads/2009/lily_metaphase2.jpg",
                               "http://www.microbehunter.com/wp/wp-content/uploads/2009/lily_anaphase2.jpg",
                               "http://www.microbehunter.com/wp/wp-content/uploads/2009/lily_telophase.jpg"))
df_img$phase <- factor(df_img$phase, levels=df_img$phase)

ggplot(df_img, aes(x = phase, y = 0, image = image)) + geom_image(size = 0.18)

And this is the resulting image:

This is what the image should look like:

Note: This is a question specifically about the behavior of ggimage. I know how to generate the correct image using other approaches, e.g. by using draw_image() from cowplot.

ggplot(df_img, aes(x = phase, y = 0.25, image = image)) +
  geom_image(size = 0.5, by="height")+
  scale_size_identity()

will produce

If by is specified, size is mapped and interpreted as npc in the data space, either as width or height, and the aspect ratio maintained for the current device (not after resizing). If size is Inf, the picture stretches to fill the whole panel.