Thanks. I was in exactly the same position as you, I looked for ages for a web app chops up images and writes HTML but there was nothing. So I built Mailrox.
You can currently save, duplicate and override emails and designs so it's easy to upload a new one over some existing slices. However the exact functionality you're talking about (swapping out images in a cell) is coming very soon. (You can pop in a support ticket/idea if there's anything else you'd like to see, I've just put your request in as a new feature request.)
Currently the images are only exportable in the downloads area (they're all neatly packaged up for you!) because image hosting might have been open to abuse, but it's certainly something we're thinking about for the future.
You can currently save, duplicate and override emails and designs so it's easy to upload a new one over some existing slices. However the exact functionality you're talking about (swapping out images in a cell) is coming very soon. (You can pop in a support ticket/idea if there's anything else you'd like to see, I've just put your request in as a new feature request.)
Currently the images are only exportable in the downloads area (they're all neatly packaged up for you!) because image hosting might have been open to abuse, but it's certainly something we're thinking about for the future.