Forums

It is all by design.
The partly visible thumb is there to show there are still thumbs on the left side.
The carousel takes the width setting for thumbnails to spread six of them over the width of the visible part. Hence the fact that portrait oriented thumbnails have gaps.
Having that all said, lazyest-slides is flexible enough to get those gaps filled. You can use a higher number (than six) of images in the visible part by using a filter. Please test a little to find the right number.
The example below uses eight images

 
function my_minitems( $items ) {
  return 8;
}
add_filter('lazyest-slides-carousel-minitems', 'my_minitems');