Why does the picture on my phone not rotate when I rotate my i phone 8?

Probably because you have rotation lock enabled. See this Apple support article—>If the screen doesn't rotate on your iPhone, iPad, or iPod touch - Apple Support

Note that not all screens rotate. The Home screen on a 5S does not, for example, and some apps will not rotate.

More than likely you enabled the rotation lock. Swipe up from the very bottom of the screen to access Control Center. If you see a red lock icon with a circular clockwise arrow button, then rotation lock is on. Press the button to turn it off.