Here are the two https images that I tried.
The first one is an attachment to the Confluence page where I've added the Github markdown macro. So it does require a login (to Confluence), but all users at this page will be logged in.
![plate schematic from Confluence attachment](https://wiki.zymergen.net/download/attachments/6951800/plate_schematic.png)
This image does not appear. The alt tag appears as a link, and if you click it, it attempts to access the page https://camo.githubusercontent.com/f07009d6b61012c8ae47eb45efe957fb735f6e65/68747470733a2f2f77696b692e7a796d657267656e2e6e65742f646f776e6c6f61642f6174746163686d656e74732f363935313830302f706c6174655f736368656d617469632e706e67 and there is an error message, "Cannot proxy the given URL".
The second one is from a google image search. This image does appear.
![plate schematic from google image search](https://upload.wikimedia.org/wikipedia/commons/f/f6/96_well_plastic_cell_culture_plate-03.jpg)
Are you saying this is the expected behavior? I am quite surprised that the Confluence attachment is not accessible, because all users have access to the attachment.