Create Thumbnail from Blob online

Hi, i stored some jpegs in an mysql-database (one field for data, one for type) and i'm able to download them and to show them in an <img> tag. But wat i want is to use the stored data to create a thumbnail to show it within the browser without creating a file - how can i pass the mysql-blob directly to a gd-function like imagecreate() or imagejpg() ?? (gd-library is working well) thank you for any answer, tommi