Yeah, do what Teal said to get to your profile then to insert a picture type:
{img}http://www.yuto.com/yuto.gif{/img}
But replace the '{' and '}' with '[' and ']'. This example would work if the picture you want to use is located at
http://www.yuto.com/yuto.gif so replace that with the address of the pic you want to use.
Don't forget, the picture must be online, not just on your hard drive.