Creates internal thumbnails for images uploaded outside of ExpressionEngine. Requires EE 2.1.4 or later.

create_ee_thumbs

<?php
	// Change your upload directory ID here.
	$upload_dir_id = 2;
	
	$this->EE =& get_instance();
	$this->EE->load->library('filemanager');
	$files = $this->EE->filemanager->fetch_files($upload_dir_id);
	foreach($files->files[$upload_dir_id] as $data)
	{
		$dir = array('server_path' => $data['relative_path']);
		if($this->EE->filemanager->create_thumb($dir, $data) == TRUE)
		{
			echo('<p style="color: green;">Thumbnail created for <strong>'.$data['short_name'].'</strong>.</p>');
		}
		else
		{
			echo('<p style="color: red;">Trouble creating thumbnail for <strong>'.$data['short_name'].'</strong>.</p>');
		}
	}

	/*
           Paste this in a PHP-enabled template and load it up.
           Will only work in EE 2.1.4 and later.
        */
?>

Hey Bob – EE does create thumbnails when you upload a file, and this function in fact just hooks into that process directly. But these are just internal thumbnails for EE's use (showing a preview on the Edit screen after you revisit the entry) -- they're not for using on the front-end of your site. The function actually has the thumbnail size hard-coded.