There are some buttons on the DS that do some special things on PictoChat. Here they are:

Control Pad: (Obviously) To go around on the keyboard on the bottom screen.
X: Press it once and you've switched to shift. Press it twice and you're on Caps lock.
Y: Switches your current keyboard thing to the next on the list ( The five symbols around the bottom left part of the screen)
A: Umm... Exactly what you think it does.
B: Works as backspace
R: Scrolls up one message on the top screen
L: If you pressed R, scrolls down one message on the top screen.
Select: A quicker way to bring the lower-most message on the top screen down to the bottom screen for you to edit. (You can change what message you bring down by scrolling up and making the message you wanna copy the one that's at the bottom of the screen)
Start: A quicker way to send your message