I just spent an hour trying to figure out why the new image I uploaded with the CMS for the Details: Header - Main Template would not update in the viewer. It would continually revert to the previously loaded image. I even checked that the new image was indeed loaded to the sever. Out of frustration I clicked save and when the CMS exited the editor there was the new image.
What are you referring to when you say "Viewer"? If you are referring to the HTMLEdito then yes, if you used the images upload and added an image to the "viewer" then you will need to click save in order for the content to update. I'm not sure how you would like this to work otherwise.