I centered and scaled proportionally an image inside a hyperlink both horizontally and vertically this way:
#link {
border: 1px solid blue;
display: table-cell;
height: 100px;
vertical-align: middle;
width: 100px;
}
#link img {
border: 1px solid red;
display: block;
margin-left: auto;
margin-right: auto;
max-height: 60px;
max-width: 60px;
}
It was tested in Internet Explorer, Firefox, and Safari.
More information about centering is here.