Hello Sami, and welcome to the Club.
Weasel has answered you as far as ELO (which incidentally, doesn't stand for "anything" the system is named after it's inventor, Arpad Elo). If you want a more detailed answer, Wikipedia does a fair job of it. Keep in mind or system is modified to factor in drawn games.
http://en.wikipedia.org/wiki/Elo_rating_system
There is no most common operation for Blitzers. For us, playing is the most important thing and any game that two players can agree upon will be allowable for scoring purposes. Each ladder varies as much in what is played as in the type of play. To answer that question better you'd have to ask it on a specific ladder communities forum.
Yes it's possible to have a custom picture. Send it to me (please take note of the size limitations) and we can add it to your profile for you.
Feel free to ask questions, someone will usually give you an answer before too long. And if you're holding off on accepting games because you feel you're not experienced enough, my advice would be don't hesitate. Your learning curve will improve exponentially against live opponents. Some ladders even have mentors available to help tutor inexperienced players if they ask for it.
Good gaming;
Paul aka steel god