Perhaps the image source is blocked in the firewall, perhaps it could be something in the url path that is matching some ad blocker. But that would be the same for all images that are attached (and gifs) not just yours.
A .gif format should be fine (although jpg is a smaller file size) can you see the image that attached this time, I can and I can see the original images of the link you posted.
I assume that have cleared your browser cache and tried again, have you tried attaching a jpg instead, have you tried a different browser?