I cannot submit new content to my website www.medicdebate.org

I get the error message "The directory files/images is not writable" as soon as I try to create any kind of content, including comments, books, pro-con arguments etc
When I use the same database on my local computer, I do not have this problem,
I did not have it some days ago, and I did not change anything on my server.
What can I do?