images are missing

[robertoostenveld]

@marlenemeyer reported that images are missing on https://www.fieldtriptoolbox.org/workshop/madrid2019/tutorial_freq/ (and possibly others). I can confirm.

In the readme we have a section on checking for broken links and images.

wget --spider -r -nd -nv -o spider.log http://localhost:4000
grep -B1 'broken link!' spider.log  | grep http > broken.log