Use instead
<div [ngStyle]="{background: 'url(/img/' + item.img + ')', width: '200px', height: '150px'"></div>
or
<div [style.background]="'url(/img/' + item.img + ')'"
[style.width.px]="200" [style.height.px]="150p"></div>
See also In RC.1 some styles can’t be added using binding syntax