Margin-right broken on width 100%

It’s better if you remove width:100%. write like this:

.photo-caption  {
            left:0;
            right:0;
            background-color: black;
            position: absolute;
            bottom: 0px;
            margin-right: 10px;
            margin-left: 10px;
            margin-bottom: 10px;
            }