It has got to be a private sexual image, so if he has taken it and shared it then it seems fairly clear and probably would still be so if it was shared with him by someone who took it (and then obviously used to embarrass / harass (edit) though the offence is made out just by posting it).
I think beyond that (for such an image) we'd really need a Court to define whether sharing it when far removed from the taker would make this offence out; I mean if that was the case anyone who shared that tweet or commented on it might be at risk.