Try this:
if( get_comment_author() == get_comment_author_link() )
echo get_avatar( $comment, $size="75" );
else
echo '<a href="' . get_comment_author_url . '">' . get_avatar( $comment, $size="75" ) . '</a>';
Try this:
if( get_comment_author() == get_comment_author_link() )
echo get_avatar( $comment, $size="75" );
else
echo '<a href="' . get_comment_author_url . '">' . get_avatar( $comment, $size="75" ) . '</a>';