Sumathy,
That makes a lot of difference then. The Launchpad theme handles things very different than other themes.
In the apps themes\LaunchpadTheme\styles\default\css (assuming you are using the default style) folder open the launchpad.css file make these changes:
}
.claro .HomeButton .home {
background-image: url(./images/zoomExtent.png);
background-position: center center;
background-repeat: no-repeat;
}
And now you need to add the png file to the images "themes\LaunchpadTheme\styles\default\css\images" folder.