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/timthumbPorS3G

Query String : w=680&zc=1&a=c&src=http://www.sienacamping.com/gestione/uploads/photo/55891_G0013488.JPG
TimThumb version : 2.8.13