how to put pic in each post?

i see that a lot of people have a picture just below their identity on the their posts. how can i do this, and is it a one shot deal or do i have to go through a process EVERY time? please help. thanks, groucho. P.S. i followed someone elses info and every time i tried to submit it said my pic was too big (even though i shrunk it to almost nothing). not to mention there's a guy with a killer boob motion pic on his I.D. (how many bytes does that take?)