A 3D version of the avatar bkub uses on Pixiv and had previously used on Twitter.

In the replies, bkub would ask TheCoolMuseum if he could use it as his avatar, which he was obviously allowed to do. From the 28th of April 2016 onward, he used this version as his avatar, and it wouldn't be until the 18th of February 2019 that he switched to something else, a low-res image of a Cheep Cheep.