You don't. Once you've squeezed data out of a picture, it's gone.
The resize/resample feature in some image manipulation programs (such as
IrfanView) can give you a larger image, smoothing out some of the jaggies as
they do so, but it will not be of as good a resolution as the original
image.
Always, when modifying an image, save to a new file - keep your original.
Val
How Do I Uncompress a Picture in Microsoft Outlook Picture Manager?