完成形のコード

<template>
  <div>
    <img 
      :src="image"
      @mouseover="imageChange()"
      @mouseleave="imageReturn()"
    >
  </div>
</template>

<script>

export default {
  data() {
    return {
        image: require('@/assets/images/off1.png'),
    }
  },
  methods: {
    imageChange(){
      this.image = require('@/assets/images/on1.jpg');
    },
    imageReturn(){
      this.image = require('@/assets/images/off1.png');
    }
  }
}
</script>