I think I've figured it out. .ljuser consists of two kinds of links - a (=mini icon) and b (=user name) - so you need to style each individually. So it's like you said first, the link wasn't actually on the image. But the entire linking item doesn't consist of one box but two different boxes.

So, thanks for pointing me in the right direction :)

My solution:

a, b {color: #1b649a; text-decoration: none; border-bottom: 1px solid #ddd;}
a:hover, b:hover {color: #073a60; border-bottom: 1px solid #073a60; }
a:visited, b:visited {color: #073a60; text-decoration: none; border-bottom: 1px dotted #073a60; }
a:visited:hover, b:visited:hover {color: #073a60; border-bottom: 1px solid #073a60;}

.ljuser a{border: none !important}