Tiled Gallery hover icon

This seems to be addedvia CSS by the theme. You can check the theme’s stylesheet here, and look for all gallery customizations. Here is the part about the glass icon:

.gal > a, .tiled-gallery-item {
	position: relative;
	display: block;
	line-height: 1;
}

.gal > a:after, .tiled-gallery-item:after {
	content: " ";
	position: absolute;
	background: url("http://demo.cssignitercom.netdna-cdn.com/preview/memories/wp-content/themes/wp_memories5/images/image-overlay-link.png") no-repeat;
	width: 46px;
	height: 46px;

	top: 40%;
	left: 50%;
	opacity: 0;
	margin: -23px 0 0 -23px;
	cursor: pointer;

	-webkit-transition: all 0.2s ease;
	-moz-transition: all 0.2s ease;
	-ms-transition: all 0.2s ease;
	-o-transition: all 0.2s ease;
	transition: all 0.2s ease;
}

.gal.zoom > a:after, .tiled-gallery-item:after {
	cursor: pointer;
	background: url("http://demo.cssignitercom.netdna-cdn.com/preview/memories/wp-content/themes/wp_memories5/images/image-overlay-zoom.png") no-repeat;
}

.collage { padding: 5px; }
.collage li { margin: 0; padding: 0;}

.gal img, .tiled-gallery-item img {
	-webkit-transition: all 0.2s ease;
	-moz-transition: all 0.2s ease;
	-ms-transition: all 0.2s ease;
	-o-transition: all 0.2s ease;
	transition: all 0.2s ease;
}

.gal:hover > a:after, .tiled-gallery-item:hover:after {
	opacity: 1;
	top: 50%;
}

.gal:hover img, .tiled-gallery-item:hover img {
	opacity: 0.5;
}

.gal figcaption  {
	background: #131313;
	padding: 8px 12px;
	border: 1px solid #2E2E2E;
	border-top-width: 0;
}

.tiled-gallery-caption {
	background: #131313 !important;
	background: rgba(0,0,0,0.6) !important;
	color: #FFF !important;
}

.collage-item { margin: 5px; }

.gallery-item { margin: 0 0 10px 0; }