This is an understandable misreading of an unfamiliar culture by the Chinese . The Tibetans, like the Indians (and unlike the Chinese) have a tradition of leaving human bodies out to be eaten by vultures. And they hack the bodies to pieces. And in some cases remove the skin, which again Indian yogins do too, or at least did in the past, not sure if they still do.

But this is for dead people, after they've died naturally, and done as a ritual. Like our customs of burying dead people or burning them in a cemetery. It helps them to pass on to the next life, to leave their previous life behind. It's also an act of generosity towards the vultures.

It’s like claiming that here in the UK we bury people alive or burn them to ashes alive in crematoria. If all you knew was sky burial and you saw people lowering a coffin into a grave, you might well think we bury people alive. But of course we don’t and it’s the traditional way we treat dead bodies. For us, this is a mark of respect just as sky burial was a mark of respect, and indeed the second most prestigious type of funeral, for some Tibetans. The most prestigious funeral of all in old Tibet and still sometimes used by the Tibetans in exile was a slow cremation designed to leave unburned bones which is used only for highly respected spiritual teachers with their bones kept as relics.

If that’s not it, then it is just fake news.

IN DETAIL

In Tibet then - well the very highest status burial is cremation - used only for highly respected teachers. Their bodies are burnt in a low temperature fire to preserve the bones as relics.

Apart from that, the highest status burial is "sky burial" in many parts (remember Tibet is a diverse country with many different local customs).

The body is hacked up after death and then left out for vultures to eat. This seems gruesome to Westerners - but in the cultural context, this both helps the individual to move on to their next life - and also it is generosity offering their bodies to the vultures.

It was the most expensive form of burial in Tibet. So the people who get these sky burials would be the wealthiest in the community.

Remember the Chinese wouldn't be able to speak Tibetan probably at the time, and they wouldn't understand the ceremony as there is no tradition like this in China AFAIK. The Chinese traditions of Buddhism have developed separately from the various Indian and Tibetan traditions for nearly 2000 years now with little contact between the two, and the Tibetans are in the lineage of the Indian traditions.

So I think the most likely explanation is that some Chinese saw Tibetans hacking a body to pieces in a ceremony, and, naturally enough, jumped to the conclusion that this was a live sacrifice, not realizing that what they were witnessing was a "sky burial".

The ancient pre-Buddhist religions apparently did originally involve animal and even human sacrifice. This has gone into the iconography of Tibetan Buddhism with gruesome images.

I've also read that the ancient animal sacrifices were turned into cakes offered on the shrine ("torma"). See The Handbook of Tibetan Buddhist Symbols

There is another historical background also, the idea of meditating in charnel grounds. There - this is something done in India, and in Tibet also - a charnel ground is a place where dead bodies are disposed of, common in both India and Tibet, where the bodies are left exposed to view for birds such as vultures to eat.

So - much of the iconography is to do with this sky burial. Yogins would make cups out of skull caps. Similarly to the way skulls were often represented in Western paintings in the past.

Aelbert Jansz van der Schoor Vanitas Still Life (Skulls on a Table) 1660 Copyright Rijksmuseum Amsterdam

It's a similar idea - reminds us of impermanence and death, and that this life is not all that there is, and that we might die at any time.

That may seem a gloomy thing to us; many modern people don't like being reminded of death.

But in Buddhism, as with Christianity in these early paintings - it is a positive and life affirming message. Encourages us to use this life in a positive way and to value what we have, by being alive, and able to listen to the teachings and practice the path.

So in the same way - some yogins in Tibet (depending on the practices they were doing I imagine) would use human skull caps as drinking bowls in their rituals, and also wear robes made of skin that they took from bodies in charnel grounds, as a constant reminder of death and impermanence. Same tradition also from India.

You also have the idea in some of the practices of offering your own body, in this lifetime, as a sacrifice.

So the human sacrifices of the past have been turned into symbolic sacrifices of the yogin himself or herself. I'm not sure how that works exactly - but in general terms anyway - it is to do with offering oneself with all ones imperfections to the path - not to any external deity, but to the process of realization and understanding and opening out to others.

So the ancient pre-Buddhist (and pre-Bohn) shamanistic ideas of sacrifice are transformed into this symbolic idea of opening yourself out to the path. It's shown with particularly vivid images using the iconography of death, destruction, and general mayhem which is easily misunderstood if you don't have the background explained to you.

Basically it is to do with understanding impermanence, that things change, and that you can't fix things permanently.

You can sometimes find happiness in your physical circumstances for long periods of time, and that's wonderful when it happens. But none of us can build a permanent beautiful home in Samsara by our own efforts that will be ours for all time.

So the idea of all these practices is as a way to recognize that the only realistic thing to do in the long term is to recognize this. And the Buddha taught a way to work with this, which involves, amongst other things, opening out to others and becoming more aware and in tune with the situations around us.

Which can then lead us to be able to relate to the many changes that are inevitable. So that, instead of seeing them as purely destructive, destroying all our attempts at fixing our present situation and creating stability, they can be something that is part of the entire process we are also working with ourselves

I don't know if these are garments made of human skin. Perhaps they are made of animal skin or fabric. But if so - that's the explanation and so of course the images would be added later, nobody has images like that on their bodies while alive.