Where is that option

Required image format
Your background must be a JPG with square aspect ratio 1:1.A resolution of at least 1400x1400 is recommended.
Backgrounds with or without zoom
A background without zoom will work as the default background in the seating plan. Your students names and badges will change its size but the background won't.A background with zoom will change its size along your students'.

Erasing a custom background
Tap and hold on it > Erase