The problem is that you set the first image through 'src' attribute and on hover added to the image a background-image. try this:
in html use:
<img id="Library">
then in css:
#Library {
height: 70px;
width: 120px;
background-image: url('LibraryTransparent.png');
}
#Library:hover {
background-image: url('LibraryHoverTrans.png');
}