It ‘s very difficult to suggest exact solution to you on this case, I need to check template setting on your site before i can give better replies, but you can try the following for now :
+ JA Onepage template with modern theme
Open templates/ja_onepage/css/themes/modern/template.css file
From
.hero-unit {
background: url(../../../images/themes/modern/header_bg.jpg) no-repeat left top;
background-size: cover;
}
Change to
.hero-unit {
background: url(../../../images/themes/modern/header_bg.jpg) no-repeat top center;
background-size: initial;
margin: 0px auto;
width: 100%;
}
Hope it helps.