The background image should show through the textarea automatically, but here's how you can mess with the opacity.
Code:
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Transparent Textarea</title>
<style>
textarea {
background-color: #fff;
background-image: url(http://www.google.com/images/logo.gif);
width: 276px;
height: 110px;
font-weight: bold;
color: #ff9900;
font-size: 1.4em;
opacity: .50; /* Standard opacity property */
filter: progid:DXImageTransform.Microsoft.Alpha(opacity=50); /* IE opacity property */
/* reducing the opacity will also make the textarea text become transparent */
}
</style>
</head>
<body>
<p><textarea>You should see the Google image behind this text.</textarea></p>
</body>
</html>