Go the other way. I tend to always produce a cd version bmp file (3000x1500) and use that as my base picture. I then tend to make the cd version a 2000x1000 and the web version a 1400x700 jpg version. each i do in photoshop. ie:
import image, image size down to 2000x1000 do any sharpening and save down to a resonable level so that I don't have a huge file. Then using history, I just go back to the base 3000x1500 bmp and image size down to 1400x 700 and the rest as stated. I've actually got this as a photoshop action so that I can automate the workflow.
I hope that helps
If I only had an hour to chop down a tree... I'd spend 45 mins sharpening the axe.