This manga image has appeared on hundreds of sites since at least 2009, and many references and comments suggest it was floating around on the internet for many years before that. An ironclad provenance is tricky, because the image has many conflicting tags and references to specific manga works, often with (as here) associated comments disputing the very same tag references often seen on other boards. Nothwithstanding, I was able to identify three different popular image boards with tags in agreement that the image is by a manga artist sometimes called Okayado. ( reference reference reference )

I find the attribution plausible because if you look at an assortment of Okayado manga art like this one, you will see significant stylistic similarities. The most significant feature in common is a signature ruddiness or blush on the cheeks of the female characters, such as in this artwork or this rather-differently-drawn centaur-girl sex comic. There is also a frequent theme of lusty centaur girls or other half-animal, half-human women.

This page of Okayado information further identifies the artist as Inui Takemaru (乾武丸), sometimes also going by オカヤド. The same page identifies a Pixiv account for this artist (using the オカヤド name), although the Pixiv account does not appear to include this particular centaur-sex image. Again, the artwork on display frequently features female characters with a blushing ruddiness on their cheeks just below their eyes.

Original post here (n.b. probably to Infernal Wonders). Image provenance by Bacchus at ErosBlog.