I spent a good amount of time trying to work this out today, and couldn't get things working using line-height or vertical-align. The easiest solution I was able to find was to set the <a/> to be relatively positioned so it would contain absolutes, and the :after to be positioned absolutely taking it out of the flow.
a{
position:relative;
padding-right:18px;
}
a:after{
position:absolute;
content:url(image.png);
}
The after image seemed to automatically center in that case, at least under Firefox/Chrome. Such may be a bit sloppier for browsers not supporting :after, due to the excess spacing on the <a/>.