I thought you were talking about a bottom border right under the slider or something…..Now I see what you’re referring to, its actually a background color. Look on line 1322 in your css for this –
.avatar, .featuredpage img, .featuredpost img, .post-image {
background-color: #F5F5F5;
border: 0 solid #DDDDDD;
padding: 4px;
}
Once you remove that background it will go away.