An example
* {margin: 0; /* used to try and remove some defaults */
padding: 0;
line-height: 1em;
text-indent: 0;
}
#sample a
{display: block; /* to allow for height/width and so on to be declared */
margin: 1em auto; /* to center the a in the page */
background: #fff url(img/png-img-300x150.png) 0 0 no-repeat;
width: 300px; /* defined by the image size */
height: 150px; /* as above */
color: #fff; /* to stand out on the background img */
font-weight: bold; /* as above */
font-size: 2em; /* and again... */
text-align: center; /* horizontally center the text in the defined box */
line-height: 150px; /* defines the line height to allow for vertical center. */
/* the value is defined by the size (height) of the image. */
border: 10px solid rgba(255,0,0,0.5);
/* just to add visual definition to the shape */
}