You need a lot of differnt icons sizes when submitting an app.
This script quickly resizes a an image (I start with 1024x768) to all of the sizes needed for iMessage extention apps.
This is based off of the script found here: http://chaosinmotion.com/blog/?p=859
HOW TO USE:
Open terminal
CD desktop (or wherever you have the script saved)
./icons.sh path/to/your/image.png