Check the tmp directory setting and check whether the IIS user account (IIS_USR?) has read-write access to this directory
Edit 1: Are you sure that your IIS website is running with this account? Can you see error messages in the IIS log? May give a clue.
Chris,
IUSR has read-write rights in folder D:\APP\Project\data\tmp and there were added the image files correctly. But the D:\APP\Project\data\files\thumbs\00 is still empty. In my development environment on my laptop the thumbnail were added in that folder.
IUSR has also read-write rights in the D:\APP\Project\data\thumbs