Broken images
It happens from time to time,
The most common cause is a break in the chain of connections between the server and the user's computer. This is completely beyond our control, and it's just a matter of waiting for the break to be fixed.
Less common is server downtime (the portrait galleries are not hosted on the same server as the rest of the site). Again this is a matter of waiting for the server to come back up again.
After either of the above, users might still experience issues due to their system (or their service provider) caching the broken images, and displaying those instead of the working ones.