On the size information tab of this post, click on the "»" symbol to go to the media_asset page for your post. Then grab the numbers at the end of the URL.
When you post your pfp AS a post, you can go down and click the arrow in the little size information tab to go to the media asset page for it and grab the final numbers in the URL. Everyone is getting confused between Post ID, Upload ID, and MediaAsset ID. It's ONLY MediaAsset IDs that it's able to accept.