Because they are modded items I do not believe the game coding will recognize it.

The game uses a system of tags for all items. As long as an item has the right tags for the type of item it is, the game and buyers will recognise it as that sort of item. This also applies to items that can change a room type.

In-game, players have no way to see what tags each item has. In the Workshop, tags are listed on each item's page and you can search the Workshop for specific tags you want in items.

For pictures, make sure the Workshop ones are tagged as pictures or photos. It's not enough for them to be located in the decorations - pictures/photos part of the store, they need the right tag.