I have now replaced the camera item with the larger item.
It's available for purchase and looks nice on the wall, but when I double-click it, it just opens a small empty window. Even when I enlarge the window, the image does not appear again, it is blank.
First of all check if the camera.item_id is set to the item id of the item: external_image_wallitem_poster_small.
If the problem then is that you can't place it on the wall, check if the type in items_base is set to "i".
We had the problem that external_image_wallitem_poster_small shared the id with a diffrent furni so we had to change the id in db and furnidata.json.
This is how we fixed that problem.
(I know it's a year after the post were made but no one answered and it's still a problem)