Kunimatsu Hiryu who works for siwaza posted on Instagram that he's the 1 who created the blue bird painting that can be seen towards the tail end of Yamino ni Mau Aoi Tori PV :
https://www.instagram.com/p/B1-XYGTA65r/
Kunimatsu Hiryu also wrote the date when Yamino ni Mau Aoi Tori PV was filmed ie. 12th June x)
Looks like I answered my own question regarding if Yamamoto Koji was the 1 who painted the blue bird painting , LOL
And DAIGO personally knows Kunimatsu Hiryu , so go figure