I have a channel set up in my teams where we asked everyone to post "proof of life" photos during the Covid-19 Lockdown period.
Most photos that were posted show a preview, even when multiple photos are posted in the communication. However, a few times, there was no preview, just the file name.
Clicking on the filename shows the image, so there is nothing wrong with the file.
The same users are able to post other images and a preview is created, so it isn't a permissions issue.
What dictates in Teams whether a preview will be created or not?