This issue is a huge pain in the a.. but I finally got it. I've seen a lot of complicated solutions. This is so simple now that I see it.
.parent {
width:70px;
height:70px;
}
.child {
height:100%;
width:10000px; /* Or some other impossibly large number */
margin-left: -4965px; /* -1*((child width-parent width)/2) */
}
.child img {
display:block; /* won't work without this */
height:100%;
margin:0 auto;
}