The woman has hardly been seen in months. Which is of course fine and proper given she's been ill, but it's important to the context of the photo.
Let's suppose for a second (and I'll be very very clear, I don't think this is the case, I think it's more innocuous than this) that she had a stroke in surgery. The unedited image, if it exists, would show, say, a droop in her face, an arm that she clearly can't use and an ankle in a brace. She isn't ready to go public with the extent of her injuries, so they doctor the photo. The photo is a lie.
Now, again, I don't think that's what happened. I still just think she's self conscious about a puffy face, and god love her for that, I would hate to have one percent of the attention on my appearance that she has.
The problem is that the image has been manipulated to the extent that the agencies can't actually rule out the outlandish possiblity of a stroke, or of them using Pippa's body while Kate's locked in the basement etc etc etc. They can't stand over the truth of the image, so they have to pull it.
And she's not a celebrity, she's the future Queen. And by saying William took the photo, he's effectively given his approval and he's the future head of state.