This code is from the Repeater Field documentation and should do what you want:
<?php
$rows = get_field('slideshow'); // get all the rows
$rand_row = $rows[ array_rand( $rows ) ]; // get a random row
$rand_row_image = $rand_row['slideshow-image']; // get the sub field value
$image = wp_get_attachment_image_src( $rand_row_image, 'large' );
// url = $image[0];
// width = $image[1];
// height = $image[2];
?>
<img src="<?php echo $image[0]; ?>" />
