Mosquito larvae are a great treat for a betta, just don't feed them to him too much or he'll refuse to eat anything else.
As for your avatar, click the "User CP" tab on the top of the board, then, in the User CP, along the left side of the screen a list of options will show up. Under the "Settings and Options" list, click the one that says "Edit Avatar". Then you'll see a screen with a box that has an space where your current avatar should be (which, in your case, should be empty), then below that a place to type or paste in the link of the picture you want use as an avatar, or below that, an option to browse your computer for a picture you want to use instead of one that is online.
Be aware that the allowable dimensions of your avatar picture should be no bigger than 200 by 200 pixels and no bigger than 24.4 KBs, whichever comes first.