A signature is whatever text you want to put (within reason, there are word limits) underneath your posts.
For example, mine is "It's like I'm sleepwalking".
You can add a signature by going to My Account and clicking on edit signature.
Profile pictures can be uploaded from your computer.
They need to be a certain size - 130x150 or something like that.
They are easy to change.
Indeed, some members change avatars daily.
Emu egg is your current rank.
Your rank is determined by your post count.
You will notice it changes as you reach different post count milestones, until you become a veteran user.
I'm not sure about the chat room.
I don't think it's working, but don't quote me on that.
If you have any more questions you might like to check out the "Frequently Asked Questions" thread in Wrongplanet.net discussion. viewtopic.php?t=32554
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It's like I'm sleepwalking