Add a Custom WordPress Image Size and Register for Use with UABB and Other Plugins

functions.php

<?php
// Add new image size for mini gallery
add_action( 'after_setup_theme', 'custom_theme_setup' );
function custom_theme_setup(){
	add_image_size( 'mini-thumb', 75, 75, array( 'center', 'center' ) );
}

// Register image size for plugins to use
add_filter( 'image_size_names_choose', 'custom_image_sizes' );
add_filter( 'uabb_photo_gallery_image_sizes', 'custom_image_sizes' );
function custom_image_sizes( $sizes ) {
	return array_merge( $sizes, array(
		'mini-thumb' => __( 'Mini Thumb' ),
	) );
}