The decision to have spacing between screenshots is preferential. 

The reason for this is that the spacing is not always uniform across platforms. When viewing the stores on the web, iPad, iPhone or Android - you will see the spacing is very different between views and devices. On the iPhone itself, it is even different between searching, ads and the app store display. So you will not be able to achieve a perfectly connected look everywhere.

If you want to add this spacing, you will need to move the device on either side about ~2%. For instance;

Screenshot 1 Phone: Left Inset ( = 2%

Screenshot 2 Phone: Left Inset ( = -102%

Editing elements in the designer

  1. Against each element you'll see the move icon on the right hand side of the element toolbar.
  2. Click on the move icon for basic element positioning
  3. In the pop out you can edit the top offset, padding above/below the element, height of the element, width of the element & the left inset.
  4. As you edit, you'll see your changes live