You should really just do this without any web application. Just use a background shell script that does the resizing automatically on any image file that you dump into a certain folder. I have similar stuff set up for naming and sorting based on geo location data.

You can even make this work for mobile by having the folder that the script runs on synced to your phone. You take a pic on your phone, it gets synced to your desktop, converted by the script, synced back to your phone.