There have been some reports floating around today of Gmail not displaying images for some messages.
In my testing I’ve been able to reproduce it with some messages in the Promotions tab, but only in the “classic” Gmail web interface. However, these same messages display images when viewed in Inbox.
This seems intentional, although no official announcement has been made. Maybe Google is launching a push to encourage migrating to Inbox? We’ll await word from the Gmail team, but in the meantime more testing is in store to pin down exactly what in the message is triggering the change.
4/5/18 UPDATE: It seems this change was not permanent, with all messages appearing to display images (even those previously not displaying them). Google has been silent about this, so we’re still unsure if this was some sort of test, an error, or something else entirely.