A TimThumb error has occured

The following error(s) occured:

The remote file is not a valid image. Mimetype = 'text/html' File = /var/www/html/timthumb/cache/timthumbwA2O5C

Query String : w=680&zc=1&a=c&src=http://duerocche.it/gestione/uploads/photo/68039_892550_10200317715127173_1869351026_o.jpg
TimThumb version : 2.8.13