in the powergallery/galleries/slideshow_dissolve/css/common.css file, edit the .pg-sd-captions selector and add a min-height option:
.pg-sd-captions {
background-color: #000;
color: #fff;
font-size:12px;
line-height:22px;
font-family:Arial,Helvetica,sans-serif;
left: 0;
overflow: hidden;
position: absolute;
text-indent: 5px;
width: 100%;
z-index: 10000;
min-height: 40px;
}