Imagemagick is a great suggestion.
Try this at the command line.
convert ./any.jpg -resize 50% ./newany.jpg (substitute a real picture name of course)
convert ./any.jpg -resize 409600@ ./newany.jpg
The first one shrinks the picture 50% and safes the altered picture to a new file.
The second one creates a new picture made of exactly 409600 pixels or about 119.5kb. You can of course alter 409600 to a number of your choosing.
Combine that with this tutorial on thunar custom actions http://www.youtube.com/watch?v=leGcvwekWvI
and you can start shrinking any picture with just a right click.
Hint.. Create a custom action with this command...
convert %f -resize 409600@ %d/newone.jpg
and give it an Appearance Condition of Image Files.
Imagemagick has a gui front end if it makes it easier. It's not in the menu, but you can launch it by just typing display in the terminal.