Script from http://deluxeblogtips.com/resize-image-fly-wordpress/

resize_on_the_fly.php

<?php 
function rw_image_resize( $attachment_id, $width, $height, $crop = true )
{
    $path = get_attached_file( $attachment_id );
    if ( ! file_exists( $path ) )
    {
        return false;
    }

    $upload    = wp_upload_dir();
    $path_info = pathinfo( $path );
    $base_url  = $upload['baseurl'] . str_replace( $upload['basedir'], '', $path_info['dirname'] );

    $meta = wp_get_attachment_metadata( $attachment_id );
    foreach ( $meta['sizes'] as $key => $size )
    {
        if ( $size['width'] == $width && $size['height'] == $height )
        {
            return "{$base_url}/{$size['file']}";
        }
    }

    // Generate new size
    $resized = image_make_intermediate_size( $path, $width, $height, $crop );
    if ( $resized && ! is_wp_error( $resized ) )
    {
        // Let metadata know about our new size.
        $key                 = sprintf( 'resized-%dx%d', $width, $height );
        $meta['sizes'][$key] = $resized;
        wp_update_attachment_metadata( $attachment_id, $meta );
        return "{$base_url}/{$resized['file']}";
    }

    // Return original if fails
    return "{$base_url}/{$path_info['basename']}";
}