Pic - picture.
post - 1. message left on a messageboard. Usage example: "to all board users - it is inconsiderate to other users to leave long posts as these cause the board to take longer to load". 2. participate in conversation in a chatroom. Example: "what was that last post? I can't scroll back. I had to reload". 3. publish words, pictures or multimedia files on a blog. 4. (non-American) mail, snail mail.
PM - private message; exchange private messages within a chatroom. Usage example: "can I pm you?", "oh goddess, I could pm you forever. You are sooooo sexy".
pml - pissing myself laughing.
ppl - people.
PS - post script; a thought added to a letter or email after the signature or sign-off.