Nicolas Giard, what syntax is used ? I try to add ' =50%' within the brackets but it doesn't work

Added in 2.0

One quick and easy way would probably be to add following markdown-it plugin: https://github.com/tatsy/markdown-it-imsize

Wiki.js already uses several other markdown-it-plugins. So this would just be one more (and only 2-3 additional lines of code).