Your code for posting images is correct but the link should have a .jpg or .png file extension. For example this one has a .jpg extension at the very end.
[IMG]https://78.media.tumblr.com/138234b496564d0a274b4e988c54d756/tumblr_onhptpTVO81ug14yyo1_500.jpg[/IMG]
Click this image:
and then copy & paste the link of the image you want to post.