This is very important to photographers: When saving a touched-up JPEG image,
unchanged image blocks should have exactly the same encoding as when loaded, so
that there is no gratuitous image degradation from repeated loading and saving.
 This could be an option on the JPEG save dialog, e.g., "Preserve original
encoding where possible."  It can easily be implemented by decompressing the
original JPEG file while saving the modified image and using the old DCT
coefficients where the image blocks match.