Looking for the script to take a screenshot, Grab is apparantely unscriptable.

To simply take a picture, you can use something like this…

do shell script "screencapture " & quoted form of "/Users/jed/Desktop/Whatever.pdf"

To get more complex and make sure you’re not overwriting an existing file, this script will mimic what the finder does (shift-command-3)…

set outputLocation to (path to desktop) --> Any colon-delimited location to save the captures
set fileName to "Picture " --> The filename prefix

--> Inspect existing files in directory for potential naming conflicts
tell application "Finder" to set currFiles to (displayed name of files in outputLocation)
set highInt to 0
repeat with tmpFile in currFiles
	try --> See if the file has a file number
		if (text 1 through 8 of (tmpFile as string)) is equal to "Picture " then
				set tempInt to (text 9 through -1 of (tmpFile as string)) as number
			on error
				set tempInt to 0
			end try
		end if
		if tempInt > highInt then set highInt to tempInt --> Records highest file # found
	end try
end repeat
set highInt to (highInt + 1) --> Create the incremented file number

--> Shell script to do screen capture
set newFilePosixPath to (outputLocation & fileName & highInt & ".pdf") as string
set newFilePath to POSIX path of newFilePosixPath
do shell script "screencapture " & quoted form of newFilePath

--> Hide the extension
tell application "Finder" to set extension hidden of alias newFilePosixPath to true