Gulp / Browsersync doesn't show Images

Question

Browser doesn't show images. If i look into the code and open the image in a new tab the following message appears: Cannot GET /app/img/b1.jpg

    var gulp        = require('gulp');
    var browserSync = require('browser-sync').create();
    var sass        = require('gulp-sass');

    // Static Server + watching scss/html files
    gulp.task('serve', ['sass'], function() {

    browserSync.init({
      server: "./app"
    });

    gulp.watch("app/scss/*.scss", ['sass']);
    gulp.watch("app/*.html").on('change', browserSync.reload);
    });

    // Compile sass into CSS & auto-inject into browsers
    gulp.task('sass', function() {
    return gulp.src("app/scss/*.scss")
      .pipe(sass())
      .pipe(gulp.dest("app/css"))
      .pipe(browserSync.stream());
    });

    gulp.task('default', ['serve']);

Thanks in advance!