This is more of a note than a question... I had a request from a client to change the image in the header of their email, so I just replaced the image archive since the HTML was all the same. It all worked great.
The strange thing I noticed is that now when I go back to old emails sent by the client and "Load Images" [in apple mail], the new header image shows up!
This was a good result in this scenario, but I could see it being problematic in other situations where someone might want to make changes to images in a template only for future campaigns.
If you're using the same campaign within campaign monitor and uploading the image archive, then the updated images will be placed at the same location on their server.
Any previous emails sent out that link back to that campaign's location on the server will pull the new images.
Best practice would be to simply create a new campaign each time, and thus prevent the overlap of linked images. (Unless it's your intention to update all emails related to one type of campaign)