You can change the image size from the file “partials/section-slider.php” ( line no: 47 )

<img class="col-md-6 img-responsive" src="<?php echo laltomato_thumbnail( 400, 300 ); ?>" alt="<?php the_title(); ?>">

Change the image size from

laltomato_thumbnail( 400, 300 );

 to your desired size.