Main tour image dimensions – how to have gallery thumbnails right under it?

Hi, What are the best dimensions for the main image for tours? In Media in your demo they are 1400×700 but when I make my images the same size, the small gallery thumbnails appear far below the main image, like here:
<a href=”https://triangle.iclawdiusdesign.com/tours/summer-magical-moment/&#8221;
Basically, how to have the thumbnails right under the main image as in your demo here:
<a href=”http://travelwp.physcode.com/tours/discover-singapore/&#8221;
Thanks