One way is to click on the Insert Image icon when you are creating your post. That will bring up a hyperlink box where you paste in the URL of the image.
I tend to save the images I use in my own Photobucket account (or there are other similar programs out there as well like Imageshack), and then I post the IMG code to here. The reason for this is that you are then not leeching bandwidth from the other site.
So far example I paste the following strings (without the * that I have included so you can see the coding)

When I post that without the *s, then all you will see is the picture:

To summarise, if you type in