I think you could try this:

<div [ngStyle]="{'background-image': 'url(' + photo + ')'}"></div>

From reading your ngStyle expression, I guess that you missed some "'"...

Also you can try this:

[style.background-image]="'url(' + photo + ')'"

import {BrowserModule, DomSanitizer} from '@angular/platform-browser'

  constructor(private sanitizer:DomSanitizer) {
    this.name = 'Angular!'
    this.backgroundImg = sanitizer.bypassSecurityTrustStyle('url(http://www.freephotos.se/images/photos_medium/white-flower-4.jpg)');
  }

<div [style.background-image]="backgroundImg"></div>

See also

• https://angular.io/docs/ts/latest/api/platform-browser/index/DomSanitizer-class.html
• In RC.1 some styles can't be added using binding syntax
• Plunker example