Where did you put the demo.png image? Generally images are put in the same folder as the .md file. This associates them with the page.
For example if you have:
pages/01.home
- default.md
- demo.png
Then you can reference that image directly with  (using markdown is the preferred approach).
You can even access this image from a sub-page pages/01.home/01.subpage/default.md with: 
However, if you want to access an image from multiple locations, you can put all the images into an empty image page like:
pages/01.home
- default.md
pages/images
- demo.png
Then you can access that image from any page with the  where image/ is actually a slug to the images page.
Note, you should really use these markdown image tags because they have support for the logic to find the image in a pages. The logic is the same as is outlined in this document about links.