page header not working on iphone

Hy guys,

I am working on a that website: http://consentidoconsultoria.es
and I am using this CSS:
.generate-content-header {
background-repeat: no-repeat;
background-attachment: fixed;
background-position: center bottom;
}

Because I want the image fixed and center bottom position. But the problem is when you use an iphone or ipad the image looks very bad.
Do you know how to fix it?

Thanks!!!