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/timthumbMK8yyC

Query String : w=640&h=420&zc=1&a=c&src=http://www.campingsanbenedetto.it/uploads/image/1460365843748.png
TimThumb version : 2.8.13