An app or script to watch folder and display new images in full screen?
The other simpler alternative is to accept that you are already in fullscreen quicklook mode, and use the following Applescript in the above Automator action to simply select the newest file.

tell application "Finder"
    set the_folder to container of (item 1 of input) as alias

    if window 1 exists then
        set the target of window 1 to the_folder
    else
        reveal the_folder
    end if

    select input as alias
    activate
    activate window 1
end tell


If you are already within a fullscreen quicklook, then effectively you are showing the newest file screen. It is akin to pressing "up" or "down" while in fullscreen quicklook on an already populated directory.

Take your first picture, enter fullscreen quicklook mode, take successive pictures.