Your profile is fair game-- you can put anything in your picture and about text, save pornography and personal attacks.

Most of us don't even care about the avatar (On sites other than this one, I use the default pic)... To me, the avatar provides a way to quickly recognize a user without having to go through the botherful process of actually reading the username. 

I doubt anyone will take you more/less seriously if you use any type of avatar. There's no big deal in keeping the avatar as the default quilt (you're not considered a noob if you do so--many experienced users choose to keep the quilt gravatar). No big deal in a pic, either. Both the pic and the quilt make one easily identifiable and distinguishable (though on high-activity sites like SO, similar quilts can get confusing)


Edit:On seeing Pekka's answer (Pekka=Discount Gucci Handbags), I do agree that a custom pic on a _question_ (for a low-rep) makes me think that the user is more interested in the community than a low rep user with a quilt. But I generally don't let that affect my votes or the way I answer.