Main image
1920px wide x 768
Avoid using any text on the image
Always start with a high res image that is 72dpi @1920px x 768px
Avoid cropping/zooming in very close on a subject.
Avoid using text within the image
Always centre the primary subject in the composition
Avoid having any key visual elements on the extreme edges of the image
If you are going to add a Insight item to the menu drop down, you will also need to have an image for the drop down menu:
Menu drop down image
144px wide x 192px high
Avoid using any text on the image