It looks like you are using a link to the imgur page that the image is in, instead of using a link to the actual image. It should end in .jpg, .png, or .gifWhat is a url?
[url=PAGELINK][img]IMGURLINK[/img][/url]
[url=https://forums.terraria.org/index.php?threads/the-tenebrae-mod.88255/][img]https://i.imgur.com/D26Tn5o.png[/img][/url]