不能使用javascript更改背景图像

当单击play按钮时，我试图更改background image，背景图像应该更改为pause，反之亦然。现在我可以播放和暂停音乐，但我不能改变图像。有谁能告诉我是什么问题吗?下面是我的代码:

function initAudioPlayer() {
    audio_0 = new Audio();
    audio_0.src = "audio/footprint.mp3";
    //set object reference
    playbtn0 = document.getElementById("audio-play-images-0");
    forwardbtn0 = document.getElementById("audio-next-images-0");
    backbtn0 = document.getElementById("audio-prev-images-0");
    //add event handling
    playbtn0.addEventListener("click", playPause);
    forwardbtn0.addEventListener("click", playForward);
    backbtn0.addEventListener("click", playBack);
}
function playPause() {
    if (audio_0.paused) {
        audio_0.play();
        playbtn0.style.backgroundImage = 'url(../img/pause-on.png)';
    } else {
        audio_0.pause();
        playbtn0.style.backgroundImage = "url('../img/play-on.png')";
    }
}

<div class="audio-controls">
     <div id="audio-prev-images-0"></div>
     <div id="audio-play-images-0"></div>
     <div id="audio-next-images-0"></div>
</div>