I think I know what the problem is. The instructions say:
If you left-click on the image first before right-clicking it, it will expand it to a full-sized image. If you right-click on the full-sized image and copy that link, you’ll get a link to an image that Discourse won’t display. If you just right-click without first expanding the image, you’ll get a link to a smaller image that Discourse will display. I’ve made that mistake myself in the past.