Selecting Main character for forum access?

[Umo]

1. What are those 3 drop down boxes? By default, the left drop down box was selected, in my case I got a number "2". What is that? Also, what about the middle and right drop down boxes? I leave those two blanks for now.

I really don't know what you are talking about. I'll edit this post in case I manage myself to find those 'drop down boxes'.
Now I know what you were referring to. Already answered by other fellow posters.

2. I'm planning to create another character later, am I able to change my main character in the future?

Sure.

3. Will I be able to change my profile picture?

No, you won't. You do need a character in order to post on the forums and profile picture is linked to that character. It will change with your in-game character accordingly, though.