Ooo! I discovered from here that I can use “url” as a parameter as follows:
data={
"type": "avatar",
"user_id": user_id,
"synchronous": True,
"url": "url_to_my_image"
}
url = "<my_site>/uploads.json"
response = requests.request("POST", url, headers=headers, data=data)
I finally get a 200!
Now, I know there is a 2nd step I need to do to actually complete this process 
, let me see what I can do…