igp_first_hashtag_as_category.php

<?php

add_filter( 'igp_image_post_taxonomy_terms', 'first_tag_to_category', 10, 4 );
function first_tag_to_category( $terms, $image, $post_id, $taxonomy ) {
	$tags = isset( $image->tags ) ? maybe_unserialize( $image->tags ) : array();
	if ( empty( $tags ) ) {
		return $terms;
	}

	$first_tag = reset( $tags );

	$term = get_term_by( 'name', $first_tag, $taxonomy );

	if ( ! $term ) {
		$term = wp_insert_term( ucfirst( $first_tag ), $taxonomy );
		$term_id = $term['term_id'];
	} else {
		$term_id = $term->term_id;
	}

	$terms[0] = $term_id;

	return $terms;
}