Scripts don't work like that. Even if you did have access to the other-persons' server to run a script, what they did to the image is not reversable. It's two images, you might as well start guessing the URL for the original-sized one.
Originally Posted by radomer
...and even then, if they're smart about it they have stashed the original version above the web root of the server making it TOTALLY inaccessible to people on the web without at least going through their login system.
Originally Posted by Custard7A
In short: you're hosed, it can't be done, there is no way to reverse engineer the image resizing and there is no way to reliably determine any possible URL to take you to the original image, if it even still exists on the server and if it exists below the www root in the first place.
I know it's not what you wanted to hear, but this is a dead issue.