css ID rule outranks a class rule ...look in a DOM inspector console at the css rules applied to your elements
you need more specific rules or less defined css
a#home.active, a#portfolio.active, a#contact{ background-position......}
or
define the url for background for each link, but set position with more generic rule that covers all and when class added will be less specific than class rule
a#home{ background-image: (url.........) }
.menu a{ background-position: 0 0}