This droplet will use OS X's internal image processing system to change the ppi value of a file without making other changes.

Copy and paste the following into Script Editor and save as an application:


on open (list_of_aliases)
repeat with i from 1 to number of items in list_of_aliases
set tImg to item i of list_of_aliases
set tPath to quoted form of POSIX path of tImg
do shell script ("sips -s dpiHeight 300 -s dpiWidth 300 " & tPath as string)
end repeat
end open


Drop a bunch of image files on it and they will be changed to 300ppi.

For the lazy people amongst you, there's a zipped version of the finished app attached below.

Ian