The "Big image" we call the "sized image" btw. You would have to modify the theme used and add something extra using partly the object model. Something like this assuming the theme does not do anything specific (you didn't name the theme used):
`
<?php
if(!hasNextImage()) {
$firstimage = $_zp_current_album->getImage(0);
if($firstimage) { // just to be sure
?>
First image
<?php
}
} ?>
`