You were linking the Imgur page, not the picture itself. The link generally has to end on the file type, in this case on .png

To get the correct URL you do a right click on the image iself and choose "Copy Image Link"

Then you paste the link between the [img] brackets.

Yo can also right click on the image and choose "Open Image In New Tab", to visualize what you are doing a bit better. In that tab there should be nothing else on the screen than the image itself.