Edit / Amended a previously annotated image in preview. I really like the annotation tool in preview, but when i annotate a jpg or png image and save it all the annotations get flattened into the image and cant be amended. 

Im aware that if you annotate a PDF it lets you save the PDF then come back to it and amend, but is it possible to annotate a jpg or png and then save it as a file format that will allow it to be amended ?

Currently if you take a jpeg on png and export as a pdf it will basically print as a pdf so it again flattens the image. 

Exporting it as a pdf first then annotating it would work, but that requires users to remember to do that first as if they dont they have to start again from scratch.
Preview app and the jpg and png file formats do not have this capability. You'll need a layered image editor like the affordable Pixelmator, Acorn, or the expensive kitchen sink Photoshop or Photoshop Elements.