Code refactoring: Improve picture support

Don't lost picture information when edit existing .xlsx files

examples/xlsx/image/main.cpp

@@ -6,12 +6,14 @@ int main(int argc, char** argv)
     QGuiApplication(argc, argv);
 
     QXlsx::Document xlsx;
-
-    QImage image(400, 300, QImage::Format_RGB32);
+    QImage image(40, 30, QImage::Format_RGB32);
     image.fill(Qt::green);
-    xlsx.insertImage(5, 5, image);
+    for (int i=0; i<10; ++i)
+        xlsx.insertImage(10*i, 5, image);
+    xlsx.saveAs("Book1.xlsx");
 
-    xlsx.save();
+    QXlsx::Document xlsx2("Book1.xlsx");
+    xlsx2.saveAs("Book2.xlsx");
 
     return 0;
 }