Background image problem on ios devices

Hi guys – hope someone can help – I am by no means an expert user, but I have built this site over the past couple of years and am pretty happy with it.

the theme is called Rock n Rolla

I have a real niggling problem with the background image though – if you test the site on a laptop using Chrome you will see the whole image of the guitarist in the background – but test it on an iphone or ipad and the image size is not correct and all you see is a little bit of the blue sky.

I have trawled the internet looking for a solution but have drawn a blank – can anyone please help me?

Thanks ??

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