如下代碼，加了幾次默認(rèn)圖沒加上，請問怎么加默認(rèn)圖？然后當(dāng)圖片加載出來之后就替換默認(rèn)圖   <el-table-column label="封面" width="100%" >           <template slot-scope="scope">             <el-popover               placement="right"               title=""               trigger="hover">               <img :src="scope.row.cover" width="180px" height="250px" > //請忽略這個圖              <img  slot="reference" :src="scope.row.cover" style="height: 80%"> //真正顯示的圖片            </el-popover>           </template>         </el-table-column>