If the image is stored as a member in the version control system:
(opt.) create a sandbox to work in --> check-out image --> locally edit and save --> check-in new image revision.
If the image is used in an item (and thereby stored as attachment):
You can "overwrite" this attachment with an updated version. This will basically "add" the new version of the image as a new attachment. The "old one" will still be available in the historical view of the item.
HTH Matthias
