I looked at the URL of this post:

https://sp.pf-img-a.mbga.jp/12007826?url=http://app-mbga.jamx.jp/assets/v2/image/clothes/avatar/avatar_71633_l.png%3Fv%3D2

The number after the slash caught my attention:
12007826

This is the game number in the Mobage system:
http://www.mbga.jp/_game_intro?game_id=12007826

This game is about VOCALOIDs, so this illustration is probably an asset from that game.

I found another site about this game. It features illustrations in this style.
https://www.crypton.co.jp/mikucolle/index_p.php