Hi tophee,
found a way to exclude the first post in a topic. Basically add :not(:first-child) to .topic-owner
.topic-owner:not(:first-child) .topic-avatar img {
box-shadow: 0 0 3px 1px #a7d5ec;
border: 2px solid #85c2e0;
position: relative;
top: -2px;
left: -2px
}
however as mentioned below, it’s a css3 selector and older browser might not support it. I don’t know how discourse browser support looks like. 
you can try it and check if it suits your needs 