HDMI not working

I’m using Debian and booting from the SD card.
The wiki says it’s possible to choose 2 display outputs, but how does that work? Connecting an HDMI display only shows the youyeetoo logo, but the system itself seems to work, holding ctrl alt del does restart it.

Which image are you using?

https://wiki.youyeetoo.com/en/YY3568/UsetheUbuntusystem