Once a mailing is in the inbox it can't be altered. A mailing sent out is unique to each recipient and each image in your library has been given a unique URL from our hosting server. A mailing received has also passed through several levels of filtering to get to the inbox and thus if it was able to be altered there would be all sorts of potential security issues.
If you're looking to send a new email out with that particular picture replaced, you can simply make a copy of that mailing from your "sent' tab on your Campaigns page and reinsert that image to the mailing.
I hope that answers your question but please feel free to let me know if there's anything else I can help with :)