First of all, make sure you added your logo a in the General Information section. On the right of your restaurant's name, click on Edit, change the logo, and save.
Then, in order to display your logo instead of the name of your restaurant on your Zenchef website, go to Online Visibility > WEBSITE > Advanced settings.
Scroll down, and click to activate in order to "Use my logo instead of the title".
Make sure to save at the bottom of the page.