I'd expect that if it is larger than the mobile screen width it would fallback to 100%. But I believe that all images should have an adjustment (the same way the font size is smaller, so should be the images if you resize them on the desktop). It should probably be always percentage based (to be responsive to the several device sizes), or have predefined sizes like google docs or gmail (small, medium, large, original). Remember that the main use case (IMO) is to reduce image sizes, not to make them larger (definitely not larger than what you can see in the screen)
I also miss this feature a lot - being able to scale down images inside the note or at least reducing its size (even if that implies losing quality on the photo). The main reason behind this is that pictures get uploaded from the iPhone with such a big size that they totally mess up the note if I want to have more info under context. Is this on Evernote's roadmap? I'm thinking about writing an app to integrate with Evernote and, at least, allow me to reduce the size of all images within a given note to a chosen width x height (not scaling them down, but actually reducing its size - better than nothing). Do you guys think that would help?