When the side preview is enabled and you click an SMTP mail message, the show images security option will show, but then immediately disappear.
The only way to make it appear is to collapse the preview pane and open it again:
The show images option will stay visible until you click that option. Once you click it, the images are presented and the show images option goes away (as expected), but when you navigate to another mail message, you will not be able to see the show images until you collapse and expand the preview pane again.
This one is kind of a big issue for us, as most staff use the right side preview pane and we have the show images feature enabled, so if a workaround or fix is available please advise and I would be happy to test it.