Ohh... geese man don't use these big names. lol j/k I wasn't quite sure what you were talking about.
Click on the 'user cp' button on the top of the page, then go to 'edit options', scroll down to the bottom and then you'll see "Other Options" and then you can see where you can change your Avatar (the picture). But the picture can only be so big, if you don't have any software to reduce the size you can just post it and I'll edit it for you down to a size that will fit.
Hope this helps,
§am.