Had to look in developer tools but what you’ve put into the add image box is
https // richmars.smugmug.com/Hobbies/Other-Bikes/Other-Bikes/i-nFJfS9R/A
What I’ve put is:
https // i.imgur.com/WuLzYyT.jpg
See if you can spot the difference. (I’ve had to break the address so the forum doesn’t do its thing.)
What you’ve done is copy the page address not the address of the image. I went to the page, right clicked on the image and chose “copy image address” then pasted that in place of the page address and got the image displayed.
Basically you have to copy the image address not the page address from the URI/address bar.