Hi there,
Welcome to SeaTheme forum.

It needs to define each grid:

1) Please upload a 4:3 image to replace the 16:9 image.

2) The new image will be cropped much on desktop layout of course. If you want to show a 4:3 image on desktop still, please use the custom CSS(“Appearance > Customize: Additional CSS”) to replace a 16:9 image:

@media (min-width: 768px) {
  [data-postid="1077"]section img {
      content:url(https://image-16:9.jpg)
  }
}

ps:
1077 is the single portfolio unique post ID, if you want to define multiple items, use the CSS like:

@media (min-width: 768px) {
  [data-postid="1077"]section img {
      content:url(https://image-16:9.jpg)
  }
  [data-postid="1078"]section img {
      content:url(https://image-16:9-2.jpg)
  }
}

Regards! SeaTheme – FAQs – WordPress Customization – Facebook