Hi Pippa,
The easiest way is to host them on an external site.
Two examples are:
Photobucket:
http://photobucket.com
imgur:
http://imgur.com/
Once you upload your image to the hosting site you will see a note regarding a "direct link".
Here's an example from Imgur:
Copy that link and write your post.
When you are ready click the "add image" button at the top of the posting window.

It will add two tags [ img ] and [ /img ] with the cursor flashing between them. Paste your link url in there and you are done.
Does that all make sense?