[TD] fix default directory for bitmaps

see https://forum.freecadweb.org/viewtopic.php?f=35&t=43981

src/Mod/TechDraw/Gui/CommandDecorate.cpp

@@ -399,12 +399,10 @@ void CmdTechDrawImage::activated(int iMsg)
     std::string PageName = page->getNameInDocument();
 
     // Reading an image
-    std::string defaultDir = App::Application::getResourceDir();
-    QString qDir = QString::fromUtf8(defaultDir.data(),defaultDir.size());
     QString fileName = Gui::FileDialog::getOpenFileName(Gui::getMainWindow(),
-                                                   QString::fromUtf8(QT_TR_NOOP("Select an Image File")),
-                                                   qDir,
-                                                   QString::fromUtf8(QT_TR_NOOP("Image (*.png *.jpg *.jpeg)")));
+        QString::fromUtf8(QT_TR_NOOP("Select an Image File")),
+        QString::null,
+        QString::fromUtf8(QT_TR_NOOP("Image (*.png *.jpg *.jpeg)")));
 
     if (!fileName.isEmpty())
     {