Looking to make an opaque image/image caption in a teaser
I did this in CSS using the content title and the opacity feature of CSS. See below (modify for your purpose)

.view-id-programmes.view-display-id-block_1 .views-field-title {
margin-left: 10px;
position: absolute;
background-color: 
white;
filter: alpha(opacity=68);
-khtml-opacity: 0.68;
-moz-opacity: 0.68;
opacity: 0.68;
-moz-border-radius: 15px;
-webkit-border-radius: 15px;
-khtml-border-radius: 15px;
border-radius: 15px;
padding: 8px;
}