You can center an element vertically with this trick:
change value of margin if you change width or height of your button.
-17px is half of height and -30px is half of width
.thumbnail-mask .btn{
position:absolute;
top:50%;
left:50%;
margin:-17px -30px;
}
and for zoom on picture you can use this:
.my-thumbnail:hover img{
-webkit-transform:scale(1.5);
-moz-transform:scale(1.5);
-o-transform:scale(1.5);
-ms-transform:scale(1.5);
transform:scale(1.5);
}
and if you want display your picture with background css property, you must have height on your container .my-thumbnail
.