In reply to Gary Taylor.

What browser and OS combination are you using?

On GNOME based Linux systems, I can hold Compose, press ~ and then n, to type "ñ" of Buñuel in the paragraph block, as well as in the Image caption, and it saves and renders properly. In Mac OS, you can probably press Option + n to do this. Windows has a character map to copy it from.

Providing ways to type characters is not the editor's responsibility, it's the responsibility of your operating system's input method to provide this, and most mainstream ones do.