Make a screen-shot and upload with a script
simply run shell command:

screencapture -i filename


this will start screencapture in interactive mode (selecting region). See screencapture -h or man screencapture

If you realy need run the screencapture command from the applescript just use:

do shell script "screencapture -i filename"


you can test the applescript from shell:

osascript -e 'do shell script "screencapture -i /tmp/filename.png"'