You can add a semi-transparent white on black caption to your images using simple CSS. The opacity of the caption background can be changed by modifying the alpha parameter in background-color’s rgba attribute.

<style>
  .image {
    width: 600px;
  }
  .image .black {
    position: relative;
    left: 0;
    bottom: 60px;
    width: 100%;
    padding: 10px 20px;
    height: 40px;
    background-color: rgba(0, 0, 0, 0.6);
    filter: progid:DXImageTransform.Microsoft.gradient
          (startcolorstr=#7F000000,endcolorstr=#7F000000) 9;
  }

  .image .black a {
    color: white;
    font-size: 18px;
    text-decoration: none;
    outline: none;
    font-family: georgia;
  }

  .image .black .follow {
    margin-top: 5px;
    position: absolute;
    right: 10px;
    top: 30px;
    font-size: 12px;
    font-family: verdana;
  }
</style>

<div class="image">
  <img src="img.png" />
  <div class="black">
    <a href="#"> Image Caption </a>
    <a href="#" class="follow"> Image Credit &rarr; </a>
  </div>
</div>

Instead of transparent backgrounds, you can even have fading gradients that transition from black to white much like the lower thirds in videos. Add this snippet to the CSS.

background: -webkit-gradient(
  linear,
  left bottom,
  left top,
  color-stop(1, rgba(0, 0, 0, 0)),
  /* Top */ color-stop(0, rgba(0, 0, 0, 1)) /* Bottom */
);

/* Gecko */
background: -moz-linear-gradient(center bottom, rgba(0, 0, 0, 1) 0%, /* Bottom */ rgba(0, 0, 0, 0) 100% /* Top */);