Would you like to change them for a particular theme or for all themes?
If it's for all themes (and you don't define a different path to images) you can replace the avatar files in styles/default/xenforo/avatars (Overwrite them, save your avatars with the same names and overwrite them).
*Edit: I didn't understand your post, do you want the avatars to be changed only for visitors or you want the avatars changed for everyone? The method I mentioned changes the default avatar for all members and guests/visitors.