Your link is to pixel.mamutut.space/p/…/621370665154999687 which is not actually the picture, but a web page that includes the picture (plus some other elements). Some web pages include metadata that lets services like Lemmy pull out the relevant image, but that isn’t present or isn’t working correctly here, so we don’t see the picture.
If you click the picture at your link, it takes you to the picture itself at …mamutut.space/…/Pu8IxA7M4OKDSRYvr01WzyNJEwv0IkEV… . If you replace the URL in your post with this one, it should work as you expect.