Why If remove round attribute from button then q-image is not displayed?

[Dozez13]

<div id="q-app">
  <div class="q-pa-md">
    <div class="q-gutter-y-md">
      <q-btn   >
       <q-img src="https://placeimg.com/500/300/nature"
    ></q-img>
      </q-btn>
    </div>
  </div>
</div>

new Vue({
  el: '#q-app',
  data () {
    return {
     
    }
  }
})

[Dozez13]

I Have already found my mistakes.