You can use your own picture.
Just find a picture somewhere on the web, and resize it with any picture program (like Paint) so that it's smaller than 100x100 pixels and no more than 16K in size.
Once you have that done, go you your user control panel (the User CP button at the top of this page), then click 'Edit Options'.
Now scroll down to where it says 'Avatar', and click the button next to your current avatar.
This should take you to a new screen... go down to where it says 'Use custom avatar', click the button that says 'yes' next to it, then select the picture from your hard drive with the 'Choose File' button... then click 'Submit Modifications' and voila! new avatar.
If your avatar picture is the wrong size the forum will tell you... I'm a Mac guy myself so I'm not sure how you'd change the size with Paint... somebody else may be able to fill you in on that part.
HTH.