So im using your typical background cover css code
-webkit-background-size: cover !important;
-moz-background-size: cover !important;
-o-background-size: cover !important;
background-size: cover !important;
background-position: left top;
background-repeat: repeat;
background-attachment: scroll;
Which works most of the time, unless i resize the window so its taller than it is wide, then this happens (it was my understanding that with size set to cover, it would just make the image taller, not create a white space underneath it)