Recent version’s of all browsers will support CSS opacity Effect., For lower versions of IE we have to use filter property.
Example for transparent Image css
<style>
.image_transparent{
opacity:0.2;
/* For IE */
filter:alpha(opacity=20);
}
</style>
<body>
<img src="../images/discover_css.gif" class="image_transparent">
<img src="../images/discover_css.gif">
</body>

Also this effect can be applicable for transparent text and Background image transparent
Example for Back ground image transpatent and Text Transparent CSS
<style>
.bgimage_transparent{
background-image:url("../images/discover_css.gif");
opacity:0.2;
/* For IE */
filter:alpha(opacity=20);
width:550px;
height:200px;
display:block;
}
.text_transparent {
opacity:0.4;
/* For IE */
filter:alpha(opacity=40);
}
</style>
<p>Non Transparent Text</p>
<p class="text_transparent">Transparent Text</p>
<p>Transparent Background below</p>
<div class="bgimage_transparent"></div>
Non Transparent Text
Transparent Text
Transparent Background below