If I remove this from your css it works (line 2439) –
@media only screen and (max-width: 1800px) {
.front-page-1,
.front-page-4,
.front-page-5,
.front-page-7,
.front-page-9,
.front-page-12 {
-webkit-background-size: auto;
-moz-background-size: auto;
background-size: auto;
}
}
Then add the background-size: cover;
mentioned earlier and it should work. It worked for me when I tested this on your site.
If you still are unable to get it working, let me know. I can login and make the adjustments for you if you want to create a temporary login.