I have an image button defined as DIV with background image:

.buttonDIV 
{
 width: 100px;
 height: 50px;
 background: url("../img/bulbButton.png");
 z-index: 0;
 position: absolute;
 visibility: visible
}

Which gives me my button:

But when I tap it in my application I got a strange shadow over it, like this:

But it should be still:

Any ideas how to elimiate this ?