problems with uploading images

Hey my name is Claudia, my wordpress (www.letteraturaitalia.it) is three years old and I have uploaded images without problems since three days ago, what happens? When I try to upload appears a message that says “this kind of files can’t be uploaded because of security problems” , this happens also with images that I have other times uploaded with success.
Can you help me
Thanks
Claudia Meneghini