In Kentico 10 I use this code in K# transformations and it works well:
<img src="{% GetUserAvatarImageUrl(UserAvatarID, UserID, 150, 100, 150) %}" alt="{% HTMLEncode(GetUserFullName(UserID)) %}" class="img-responsive">
I think you can replace UserAvatarID with null and UserID with NodeOwner like this:
<img src="{% GetUserAvatarImageUrl(null, NodeOwner, 150, 100, 150) %}" alt="{% HTMLEncode(GetUserFullName(NodeOwner)) %}" class="img-responsive">