Are those images uploaded before or after switching to this theme ?
Images uploaded before changing to this theme won't get the size name of
slider-img which is registered in theme's function
add_image_size( 'slider-img', 1200, 500, true );
This size is used as the conditional check in
slider.php, along with the
if ( $image >= 960 ).
while ( $slider->have_posts() ) : $slider->the_post();
if ( '' != get_the_post_thumbnail( $post->ID ) ) :
// Now let's check the image.
$image = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'slider-img' );
// If it is greater than 960 in width, let's skip
if ( $image >= 960 ) :
Disregard the "let's skip" comment there, I think it's a typo. The intention is to not do anything if image size is less than 960. But the
$image is pulling from
slider-img size which is cropped exactly to 1200x500. The 960 check is to prevent in case user upload too small image to look good in slider.
In short, delete images from pages that you set to be featured, and reupload them. To look good, make sure it's 1200x500 or bigger.