This is just a conflict with your template css. Just go to plugins/content/jw_simpleImageGallery/tmpl/Polaroids/css/template.css at line 27 and change this:
li.sig-block a.sig-link img.sig-image, li.sig-block a.sig-link:hover img.sig-image {
background-position: 50% 50%;
background-repeat: no-repeat;
border: medium none;
display: block;
height: 100%;
margin: 0;
padding: 0;
width: 100%;
}to this:
li.sig-block a.sig-link img.sig-image, li.sig-block a.sig-link:hover img.sig-image {
background-position: 50% 50%;
background-repeat: no-repeat;
border: medium none;
display: block;
height: 100% !important;
margin: 0 !important;
padding: 0;
width: 100% !important;
}
