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.
The object of opening the mind, as of opening the mouth, is to shut it again on something solid. –G.K. Chesterton
See Mediocrity in its Infancy
It's usually a good idea to start out with this at the VERY TOP of your CSS:
Seek and you shall find... basically: validate your markup | view your page cross-browser/cross-platform | free web tutorials | free hosting