Are you copying the URL or copying the share links? When you upload it to Imgur, next to the box that says copy link, there are three dots. When you select that, a menu will drop down and you want to select "get share links" and copy the one made specifically for forums.
The link you're using is the link to the imgur album; you need the direct link to the image for the img tags to work. There should be options to acquire the direct link through imgur's interface, but depending on your browser, you should also be able to just right click the image and "copy image location."
i use ios on mobile so I can only speak on that, but if you tap and hold down your finger on an image, a context menu will come up. I tap "open image in new tab" and then copy the url of the image from the address bar.