fix screenshots

assets/js/src/newsletters/send.jsx

@@ -92,8 +92,10 @@ define(
       },
       saveTemplate: function (response, done) {
         const iframe = document.createElement('iframe');
+        iframe.width = 660;
         iframe.src = response.meta.preview_url;
         iframe.onload = () => {
+          setTimeout(() => {
             html2canvas(iframe.contentDocument.documentElement).then((thumbnail) => {
               document.body.removeChild(iframe);
               MailPoet.Ajax.post({
@@ -110,6 +112,7 @@ define(
                 },
               }).then(done).fail(this.showError);
             });
+          }, 500);
         };
         // just to hide the iframe
         iframe.style.cssText ='position: absolute; opacity:0; z-index: -9999';