You could add a wrapper to the images and use an after selector.
SEE DEMO
CSS
.watermark {
position: relative;
}
.watermark:after {
background: none repeat scroll 0 0 rgba(255, 255, 255, 0.8);
bottom: 0;
content: " ";
left: 0;
position: absolute;
right: 0;
top: 0;
}
HTML