The dialog tree command [u] [/ u] [img] [/ img] has no effect
Be careful, the commands and syntax used by GDevelop are from bondage.js and are not the same as YarnSpinner. Only use commands from the wiki articles/examples, not from the YarnSpinner documentation 
OK, anyway, thank you for your reply
Commands must be like “” encapsed in “<” and closed by “>” without quotes.
https://www.gamesigniter.com/yarn-dialogue-in-gdevelop-5/