I think it’s normal because when you’re on the article, the link refers directly at the image into same folder.
Your HTML Code: <p><img src="helicopter-hill.jpg" alt="Jen at Helicopter Hill" /></p>
When this article is view on homepage, it refers at the homepage and cant find the image. Image is not on the root of homepage, but into posts/craigieburn* folder!
Attempt to deplace your image into the static/img folder, and link in your src as: 
Normally, you will obtain good code HTML for all views!
Okay, that makes sense and I can see that will work.
If I want to use Leaf Bundles so that I can keep the images etc. of each post together, is there a good way for me to link to those assets without it breaking?