ok you nearly have it right.
on that imageshack page you have there down the bottom right of the screen there is a bit that says “Links to share this image” click the arrow.
Then where it says “direct link” copy whats written there.
Go to the forum, typing your message above where you type are some buttons click “Img”
this will put img and img both surrounded by brackets.
Click between the sets of brackets and paste what you have copied like so
If you have done it right it will come out something like this