CSS background image transition

HTML

<div id="vue">
<div class="cat" :style="`background:url(${toggle ? img1 : img2})`" @click="toggle = !toggle">
</div>
</div>

CSS

.cat {
  width: 500px;
  height: 500px;
  transition: background 1s;
}

JavaScript

new Vue({
	el:'#vue',
	data(){
  	return {
    	toggle:false,
    	img1:'https://cdn.pixabay.com/photo/2015/03/26/09/47/sky-690293_1280.jpg',
      img2:'https://cdn.pixabay.com/photo/2015/09/09/16/05/forest-931706_1280.jpg'
    }
  }
})