This is a frequently asked question.
The answer is both simple and complex: HTML.
While Fargo does most of the HTML-generating for you, if you want to add HTML elements to your posts, you can. Fargo will just ignore them and pass them through. Then if someone views the page in a browser, they will show up in all their glory.
And, here's the punchline -- you can add images using HTML. And that's how we do it.
You can read about the <img> tag here.
Since Fargo uses Dropbox and Dropbox allows you to save images, you can save the image using Dropbox. Thanks to Jeffrey Kishner for this part of the howto.
Enter this text into your post, substituting [url] for the actual address of the image.
That will put the image in the post.
If you want to have it appear floating in the right margin of your post, use this:
<img src=[url] style="float: right;">
It's nice to add a little space to the left and below your image:
<img src=[url] style="float: right; margin-left: 25px; margin-bottom: 10px;">
When you come back to your post you may see the actual image instead of the HTML for the image. If you want to switch it back, type Cmd-` and do it again to toggle it back.