图片需要转化成base64，然后在下载。

  // 图片转化base64
    getUrlBase64(url) {
      return new Promise((resolve) => {
        let canvas = document.createElement("canvas");
        let ctx = canvas.getContext("2d");
        let img = new Image();
        img.crossOrigin = "Anonymous"; //允许跨域
        img.src = url;
        img.onload = function () {
          canvas.height = 300;
          canvas.width = 300;
          ctx.drawImage(img, 0, 0, 300, 300);
          let dataURL = canvas.toDataURL("image/png");
          canvas = null;
          resolve(dataURL);
        };
      });
    },
    //  下载图片
    getQrcode(row) {
      this.loading = true;
      var name = row.reportNo;
      this.getUrlBase64(row.twoCodeContent).then((base64) => {
        let a = document.createElement("a");
        a.style.display = "none";
        a.download = name;
        a.href = base64;
        document.body.appendChild(a);
        a.click();
        this.loading = false;
      });
    },