I would like to write an app for Evernote that uses the annotation feature available for images.
As an example, the file below was annotated in Evernote and saved to disk.
If you open this image in Evernote or Skitch, you can modify the annotation, as if it was a vector file format (like SVG).
But the magic is that you can also open the same image file in any(!) image viewer as a normal PNG and you can still see the annotated image (magic !) but and you cannot edit the annotation.
So the secret behind the magic - in my understanding - is that this