How do you run function on all clients when a new client joins?

I'm doing some custom stuff where the player has a flag and they can create their own flag image as a png file and load it as a sprite on their flag when they join the game. They just paste the path to the file in a text field and it will add the image to their flag. I have that working good for people that are connected, but when someone connects later, I am having a heck of time figuring out how to get all clients to update the new player. I am already sending it across the network as a byte array and it is updating all of the connected clients. When a NEW client connects, all I want is for the server to tell all of the existing clients to run the same command again, since they all have the path to the png file that is local on them, that would update the new client. Any ideas at all are MUCH APPRECIATED!