Esto parece funcionar.
Utiliza tu código para capturar la imagen, aunque lo he hecho para que cualquier usuario pueda usarlo.
Luego utiliza una versión editada de un script que encontré en el sitio web macosxautomation para eventos de imagen
set homeFolder to path to home folder
set imageName to "shot.png"
set saveFolderPath to homeFolder & "Desktop:" & imageName as string
do shell script "screencapture -Wimxo " & quoted form of POSIX path of saveFolderPath
--set the target_path to the saveFolderPath as Unicode text
delay 1
try
tell application "Image Events"
-- start the Image Events application
launch
-- open the image file
set this_image to open saveFolderPath
set type_ to file type of this_image
-- save in same file with icon
save this_image as type_ in saveFolderPath with icon
-- purge the open image data
close this_image
end tell
on error error_message
display dialog error_message buttons {"Cancel"} default button 1
end try