Avaitor's image didn't show because he hotlinked it from a third-party website. Farerb's image showed because he uploaded it to an image-sharing website and then shared it. Sometimes when you try to embed images from websites that don't allow image sharing, they don't show. That is a preventative measure done by some websites that don't want their bandwidth exploited and their images shared.
So, I uploaded a photo to Imgur, and I tried posting it in the TLM thread with the img tag. Even taking the "s" off https, and removing http altogether, I still got that little box. How do we properly use the img tag when we post a photo from an image -hosting site?
You need to post the BBCode that Imgur gives you to embed the image and not any of the other links. The image won't embed using any of the other ones.
After you upload your image on Imgur, you hover over it and an option appears that says Copy link with an ellipsis. You click on the ellipsis (and not on the phrase "Copy link") and more options appear. Then you select Get share links which will give you several Share Options. You go where it says BBCode (Forums) and you click where it says Copy link.
Your code needs to look like this (but bookended with img tags):