Great guide! Only one thing to add: Make them as small as possible!
It's like running an intro competition among the other hackers... after resizing & compression my avatar shrinked from 120kb to 20kb - so download speed is now 1/6! 120kb was way too much considering the fronpage of PeachyBBS is only 60kb (=text).
Just noticed that it's very painly to open PeachyBBS with my laptop using my cellphone as a modem... and there must be some users using 56kbit modem here? Hopefully not too many.