No. After it is sent it is gone from Constant Contact.
The only change you can make that will affect a sent email is to change an image that you have hosted on your own. (Don't try this with Constant Contact hosted images). You can substitute on your hosting a picture of the exact same name, an preferably the exact same size. Anyone who opens or re-opens the mail after you make the change will see the new picture.
"...will see the new picture" - but not if the pictures allready are downloaded at recivers end. Outlook and many other mailprograms downloads mail and the entire content and dosn't ceck to see if anythings is updated/changed. So chances are that no one will ever see the changes (and most have show pictures turned off anyway)