Usually when people use cover value for background-size, they use an image of size 1920 x 1080 which the resolution of typical large screens (excluding retina displays: 2650 x 1600).
But this makes the image size large, so in order to reduce the size you could optimize the image in fireworks by smoothing the image after reducing its quality a little. This way the images will remain sharp when down sampled and displayed on the usual small screens but stills retains enough quality on HD screens.