So, we all know that Issun stole one of his grandfather Isshaku's paintings and passed it off as his own.
Right? And we also know that Isshaku was there when Chibi fought Akuro. Right? And we also know that,in the beginning of Okamiden, when you use power slash on a painting in Susano's house, a painting shows up. Right? Plus, it is also mentioned that Susano was trying to hide the painting. Right? Well, my headcannon is that after the defeat of Akuro, Isshaku painted a picture of Chibi fighting Akuro. It's not exactly something you can just FORGET. And later, when Issun stole one of Isshaku's paintings, it was (conveniently) the one of Chibi beating Akuro. Issun gave it to Susano, who, at the time, would have loved a painting by a "wandering artist." However, when Kuni came along, Susano noticed the resemblance between Kuni and the boy on Chibi's back. To avoid any perplexing questions, Susano would have hidden the picture. I came up with this headcannon after examining the picture and finding that it looked suspiciously like Kuni riding Chibi.