[Responder] Alastair Campbell [Definition of authors] Recommend changes (see comments field)
Cheating slightly and reading other's answers: - I agree with Jutta, if you are creating web content you are an author. - If someone uploads a picture from their mobile, yes, they are creating web content, that seems right to me. There is a chain of authoring rather than pointing the finger at one tool, but the original picture taker is effectively an author. There may be others in the chain, but if that interface doesn't provide a mechanism for applying alt text and it gets immediately published, it probably shouldn't claim ATAG compliance...