Questions about villager homes

Not sure about flowers, but:

No, they won't put up other villager's pictures.
No, they won't change their wall or floor, and if they can, I have yet to see proof.
Yes, you can switch their music players and music.

Yes, they can put flowers on their tables! Same with shells lol

did neone try sending villager's picture multiple times in one letter? or when they request you to replace a furniture in their house and you choose the picture frame?

I would lovee to place villager pics in their houses

After ransacking my villagers' homes early in the game, I am slowly reordering and giving back their original furniture. They always like it and often put it in the same spot it was originally. A google image search almost always reveals a picture of the original house. When I get a chance to visit, I'll swipe a piece of furniture I don't like and then mail something better right away. They will put the original shirt back on too, but you might have to send it more than once.

They will take flowers and shells, but you can't hand over a pic or attach it to a letter for them. I thought any music player would play the same song, but someone below says otherwise I'll try to give them music now! I've never tried the clamshells either. I tried for a month to get Eunice to give me her round carpet - it was the last one I needed for my catalog. No matter how often I sent her a replacement she never budged. Not even when I gave her a new carpet for her birthday! I don't think it's possible to change it.

nah, you definitely cannot change wallpaper and floors.