.my-acm {
    background-image: url(images/acm-black.png);
    background-size: 70%;
    background-repeat: no-repeat;
    background-position: center center;
    vertical-align:-0.2em;
}

.my-acm:hover {
    background-image: url(images/acm-color.png);
    background-size: 70%;
    background-repeat: no-repeat;
    background-position: center center;
    vertical-align:-0.2em;
}