Hello @melanieb,

You can disable the header image in the home page by going to Home Page edit screen .

First of all go to Admin Panel > Pages > Home page >Theme Setting > Image > Image setting and uncheck the option Check to Disable Banner Image Area.

Screen shot :

https://image.prntscr.com/image/OeTDap-QSwWrfLWGQNDZUA.png

Now to add the portfolio, testimonials and food menu on dashboard you have to activate the jetpack plugin which is recommended by theme itself .

Also for any confusion you can refer to theme official documentation given below .

http://themepalace.com/instructions/themes/restaurantz-pro/

Hope this helps .

Best Regards!!

Have a good day .