Default Avatar url

[Destroy666]

Or, even simplier and properly scaled, use format_avatar() in 1.8 which was made exactly for this:

$avatar = format_avatar($referral['avatar'], $referral['avatardimensions']]); // also select avatardimensions column in your query, you can provide 3rd argument for max dimensions other than default
echo $avatar['image']; // src
echo $avatar['width_height']; // width and height attributes