this style in style.css (line 323) is restricting the image:
#wrapper img {
max-width:700px;
height:auto;
}
remove it, but check for unwanted side effects in other parts of hte site, or overwrite it for just the one page with:
.page-id-53 #wrapper img { max-width: 100%; }
working with a browser inspection tool such as Firefox’ web developer add-on might help to identify which styles are responsible for formatting which element.