you need designer to do a double image. Place the main image and mouseover image in one image. like this.

Now use this CSS code. Note the use of the Height and width. also the overflow and background attachment.
.download {
background:url(images/download_btn.gif) no-repeat 0 0;
display:block;
height:50px;
margin-left:8px;
outline:none;
overflow:hidden;
text-indent:-9999px;
width:127px;
}
 

.download:hover{
background:url(images/download_btn.gif) no-repeat 0 100%;
}