It was a picture of Kate and her children that they labelled as being of Kate and her children, and captioned 'Happy Mother's Day'.
I can see how it would be an issue if they aged Wills up a bit through editing and said 'here's the king, fit and well' or whatever.
Royalty have literally always manipulated their image - back to portraits of Henry VIII and beyond. The technology has moved on, but the need to portray whatever version of 'royal' or 'strong' or 'majestic' etc goes right to the root of who they are and what function they serve as an institution. Kate has been put in the role of picture perfect princess and that's what they need her to be, and what they need to portray.
People couldn't just accept that she had surgery and was recovering - it was all clamouring and conspiracy and insisting that Wills was trying to get rid of her or she was trying to escape, or worse that she'd been harmed in some way. So they felt pressure to release a photo and more pressure than usual probably to make it one where she and the kids looked happy and well and not like anyone was being coerced to be there or was covering facelift scars or a black eye or whatever else.
It was pretty bloody thick to think they'd get away with it maybe, given the level of speculation, but I guess I just can't see where the malicious intent was.
Plus Kate doesn't, and never will, 'lead the country'.