Please, if you don’t mind. I never knew this.
No problemo, it's pretty easy.
When you want to post an image hosted on the TF.com server, first, you'll click on the 'insert image' link, like this:
Which uploads a window like this:
Where you can either upload an image from your local machine, or, by clicking the 'From URL' take, copy an image from another server to the TF.com server.
Once you do that, your post will look like this:
If you wish to embed the image into the post (where it shows up in the post instead of simply being a link that the reader is required to click to view), just click the 'go advanced' or 'preview post' tab at the bottom of the post:
where you can see and copy the link (typically by right clicking on the link) and paste it as an image by wrapping it with [ img ] and [ /img ] (without the spaces).
Clear as mud?