Can I put a big photo of my restaurant across the top of my page?

Yes, on the Photo Hero layout. Open the Theme gallery and press Apply on Photo Hero. Then go to Settings → Branding & logo and set a Header photo: a wide 3:1 crop, 1600 by 533 pixels or larger, and press Save branding. Without a header photo that layout shows a colour gradient in your theme's colours instead. The other layouts do not have a header photo slot. The Design & appearance tab in the docs has the guide.