The png's for the chat balloons are located here:

/usr/palm/applications/com.palm.app.messaging/images/chatview

Look for chat-balloon-right.png (your chat) and chat-balloon-left.png (friend's chat).

Edit them using your favorite image editor.

Now uploaded them to where ever. Copy the image location.

Go to terminal and type:

cd /usr/palm/applications/com.palm.app.messaging/images/chatview

sudo wget (url)

Now you just need to change the name.

Type:

sudo mv (filename of url) chat-balloon-right.png

Do the same for the left chat balloon.

Now type:

sudo luna-send -n 1 palm://com.palm.applicationManager/rescan {}

Kudos goes out to DJPeterman for his help with the padding <3