Give this a whirl:
.is-sticky img.menu-image-title-hide {
background: url(http://www.spiritvoyage.com/blog/wp-content/uploads/Screen-shot-2011-12-06-at-2.56.08-PM-150x150.png) no-repeat;
width: 150px;
height: 150px;
padding-left: 150px;
}
Should do the trick. It moves the original src image out of the way and adds a background image. As far as your user is concerned it’ll just look like it flips from one image to the other.
It works because the class .is-sticky is added to the header on scroll by js.
solved Replace item in menu using only custom.css in WordPress