Why some images aren't automatically downloaded?

[zogstrip (Régis Hanol)]

I kinda replied here

Firefox doesn't support WebP image format

The issue is that the linked image is a WebP file which is currently only supported in Chrome. Here’s the link to the related bugzilla issue: https://bugzilla.mozilla.org/show_bug.cgi?id=856375.

The image you linked is a WebP file. Since it’s only supported by Chrome right now, we don’t even consider it an image.