[noypiscripter]

Try this:

.username[href^="http://www.yoursite.com/member.php?12345-"]:before {
    content: '';
    width: 16px;
    height: 16px;
    display: inline-block;
    background: transparent url("/path/to/the/image.png") no-repeat 0 0;
    background-size: cover;
    margin-right: 5px;
}

Change 12345 to the corresponding userid and of course, the domain name, image dimensions and the background image url.

Note: Do not remove the hyphen after the userid. That ensures 12345 will not match 123456.