I don't think images that may be sexually explicit are automatically porn. The definition of Porn as you posted states that it's especially those having little or no artistic merit so it can be asked who decides what has artistic merit and what does not? If someone draws two people having sex with the intention of it being purely artistic that wouldn't stop someone else from using the image as porn.

I think it would be very hard to draw a line between porn and art unless it's very obvious that it is intended to be one or the other. Even in that case though some may think of it as art, or porn.