Take a look around line 1076 in your css and add max-width: 100%; –
.slider-wide .wrap {
color: #fff;
margin: 0 auto;
max-width: 100%;
padding: 200px 0;
text-align: center;
width: 100%;
}
Then add width: 100%; to .flexslider .slides img
This code actually comes from the Genesis Slider Plugin’s css, so you’ll need to add it to the style.css file to override it like this –
.flexslider .slides img {
width: 100% !important;
}