I have been playing around with this as well. I couldn't do anything with the tombstones once I got them to a community lot. Even when I added them to a sim's inventory I still couldn't do anything with them. The stone could be highlighted in the inventory but it remained greyed out and immovable. I ended up taking them back home. :(
Seems that everyone is having similar but not exactly the same problems with the tombstones. I was able to carry them in inventory and unload them from inventory in the community lot, but they wouldn't stay in the graveyard, kept showing up at the curb when I re-entered the lot later on. So whenever I wanted pictures of a family member mourning a loved one in the graveyard I had to move it there from the curb every time. Eventually I gave up, put it back in inventory and carried it home where it's sitting on the flat roof of the house and bothering nobody. It never occurs to me to go up there and check on ol' George to see how he's doing. *Makes mental note to look in on him.* (Edit) I just remembered I've got a residential lot with a chapel on it. If there's room, I'm going to add a small graveyard to that one for now.
