Summary: You'll have to disable "@" for username tagging otherwise this will look odd and not display correctly. This enhancement is for xf v1.2 and above.
You can disable the "@" by going into Admincp >> Options >> User Alerts and then unchecking the box (screenshot Below)
Install: Hopefully i did this correctly simply copy and paste the following in EXTRA.CSS and then upload the icon to your /icons folder.
Code:
.message .messageText .username {
background: url("@imagePath/xenforo/icons/tag.png") no-repeat left center transparent;
height: 13px;
width: 13px;
margin-left: 3px;
padding-left: 15px;
}