You’ll need to have the image set as Image ID
then use something like:
<?php
$attachment_id = get_sub_field('illustrations_image');
$size = "full"; // (thumbnail, medium, large, full or custom size)
$image = wp_get_attachment_image_src( $attachment_id, $size );
$attachmentinfo = get_post( $attachment_id );
$alt = get_post_meta($attachmentinfo->ID, '_wp_attachment_image_alt', true);
?>
<img src="<?php echo $image[0]; ?>" alt="<?php echo $alt; ?>" />
You’ll need to edit it to go in amongst your loop but hopefully you’ll get the jist.