This walktrough feature i think the more intuitive way to name it is "XMB In-game manual"... that is enabled by default in "PS1 classics" (category P1), and "PSP minis" (category MN)... and you proved is partially working by booting a PS3 game with his CATEGORY changed to "MN"...
I think is managed by photoviewer_plugin.sprx module and at first i thought that is "acitaved by default" for this two categories (MN and P1) <--- edit: this is right, you proved it enabling it just by changing the category, there is no flag for this
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Edit:
Ohh sorry, you was talking about using a folder with pictures and display them from "XMB in-game" photo column ?
This is done with the same module i said, but i never noticed if is allowed in games