When using widgets (which is also the fallback for the filedialog in labs), you can set folder to be QStandardPaths::standardLocations(QStandardPaths::PicturesLocation).last() (https://doc.qt.io/qt-6/qfiledialog.html#setDirectory). This will inform the iOS QPA plugin to show an image picker dialog instead of the document picker.
With the new FileDialog in Qt Quick, this doesn't work anymore:

   FileDialog {
      id: fileDialog
      //property string pictures: StandardPaths.standardLocations(StandardPaths.PicturesLocation)[StandardPaths.standardLocations(StandardPaths.PicturesLocation).length - 1]
      property string pictures: "assets-library://"
      currentFolder: pictures
      onAccepted: {
          print("path:", currentFolder)
          print("pics location last:", pictures)
          print("Image selected: " + fileDialog.fileUrl)
      }
   }

(Note that StandardPaths.standardLocations(StandardPaths.PicturesLocation)[StandardPaths.standardLocations(StandardPaths.PicturesLocation).length - 1] translates to the URL "assets-library://" on iOS. But because of another bug in StandardPaths, you currently need to use "assets-library://" directly.