Hi,
Christian Graus is correct.
Try to add ImageButton control instead of normal button.
Anyways,If you want to use normal button ,then write some css
this is just an example.change the values as you wish.
.button{
border:none;
cursor: pointer;
background-image: url(../images/btnbg.gif);
background-repeat: repeat-x;
background-position: left top;
width: 81px;
padding:5px;
color: #FFF;
}
Happy Coding....