This image is known as Guriin Neesan, in Japanese: "グリイン姉さん" (lit: Green older sister, since Guriin is the Japanese pronunciation of Green); in Japanese, Neesan or Oneesan (its more formal version) is an honorific used to respectfully refer to a woman older than you.

These images appeared on the now defunct website Rotten.com and quickly became popular on Futabachan, an anonymous Japanese forum that inspired the well-known website 4chan.
The image supposedly appeared and became popular in 2001.
There is no information available about who this woman was, what caused her death, or why the corpse looked the way it did. Some users with knowledge on the subject have said that the greenish coloration may simply be due to the reaction caused by bacteria and other components of the body.
But another theory seems to be the answer: suicide by hydrogen sulfide.
This woman probably committed suicide with hydrogen sulfide, hence the greenish skin. This hydrogen sulfide reacts with hemoglobin in the blood, generating something called "sulfohemoglobin." This reaction, as already mentioned, causes greenish (or sometimes blue) pigmentation. The corpse shows no signs of decomposition but rather burns caused by a chemical, and the eyes show irritation consistent with this.

A disturbing detail is that Japanese users received this image with "affection." On the internet, there are multiple fan arts of the corpse, and the name itself seems to be a nickname that, unironically, seeks to be affectionate. It also has a cosplay on TikTok and a game in which our character escapes from a giant Guriin Neesan who chases us and tries to catch us with her tongue.