Strange image on Mac shutdown

Hello!

Not much for inconvenience, but nevertheless - when I shut down my mac mini I get strange image on display.

Today it was login screen, some days ago it was image of Safari with fullscreen youtube video.

The image is inactive, so no mouse or keyboard response. Then, in some time mac shuts down.

Again, it is not inconvenient, but quite annoying.

Any one got this?

Thanks