Change Page Header to image

At present on each page there is a grey background behind the page tile, I would like to change this background for an image.
If this is possible:

What size does the image have to be?
Is it possible to have a different image for each page?

The final question is what do I need to do to change this?

I am using a sento child theme.

Thanking you in advance for your support.

The page I need help with: [log in to see the link]