Using display from cronjob

[Musbacked97]

Hey there,

i managed to get my display running.
The display is connected to a raspberry pi with spi.

When starting "fbcp-ili9341" i could see the desktop.
With a keyboard, connected to the pi, i could open and view picture with gpicview.

I tried to change the pictures with a cron job, but don't get it working :(
gpicview allways returned Cannot open display:.

Does anybody know how to solve this?

[Musbacked97]

posted the question here, because it seems like a general linux question