png images in README generate error when serving site

[thibautjombart]

I was re-adding some static images to the README.Rmd (screenshots of PRs), which might help github novices. I am surprised these generate the following error:

> blogdown::serve_site()

Attaching package: ‘testthat’

The following object is masked from ‘package:devtools’:

    setup

Building sites … ERROR 2018/06/06 17:20:45 Failed to read data from img/shot-fork.png/shot-fork.png: Data not supported for extension 'png'
ERROR 2018/06/06 17:20:45 Failed to read data from img/shot-pr-2.png/shot-pr-2.png: Data not supported for extension 'png'
ERROR 2018/06/06 17:20:45 Failed to read data from img/shot-pr.png/shot-pr.png: Data not supported for extension 'png'
Total in 117 ms
Error: Error building site: logged 3 error(s)
To stop the server, run servr::daemon_stop("140025127766648") or restart your R session
Serving the directory /home/thibaut/dev/recon/learn at http://127.0.0.1:3633

I could live without these screenshots, but I'd rather have them there if possible. Any idea what's going on?