What is the URL to display a custom avatar??? (read, then it'll make more sense)

[Velocd]

With the following absolute link:

avatar.php?&userid=$post[userid]

I can display any user who is using their own avatar from another website or that they uploaded. For example,

https://vborg.vbsupport.ru/avatar.php?&userid=1

Will give you FireFly's avatar

BUT--what is the link to display a custom avatar, the ones located in your avatars folder. Because avatar.php?&userid=$post[userid] doesn't work

(quick example of this is on my forum my avatar is uploaded, found here http://www.diffusion4.com/avatar.php?&userid=1 ,but this members avatar is a custom avatar: http://www.diffusion4.com/avatar.php?userid=20 --see it doesn't work)

So, what I need is a link that I can insert into my templates and will display either an uploaded/website linked avatar OR a custom avatar.

Anyhelp??? ^_^