You want to know what's behind Isabella's locked door.
Whether the picture that hangs in Isabella's reading room, as OFT suspects, I don't know. It could be possible if the picture is hanging in front of a curtain. If not, then it is not a picture and left-right we see the frame of another door.
But I don't think it's children's shoes, more likely jumping boots. Considering that in EP2, when the MC visited Isabella, he saw a person in uniform, it makes more sense.
If the picture was hanging in Isabella's reading room, that makes me wonder, because the two screenshots below would indicate that. But make up your own minds about that.