You can just update the images (we'll keep the history) by dragging an updated image onto the old one.

(When editing the Mock.)

It will update the old image, I want to keep the old image so we can track the changes on the design. maybe grouping of multiple image into one and not uploading a new one. I think this way its much easier to look at the changes we did on the design.

Old images are kept in the Mock History under the Mock. You can click on and review old comments anytime.

Older versions of an image are available in the table below. For example, this shows iterations to a high quality professional picture of a cat:

In this case, I initially uploaded version 1 (the bottom version), and got some feedback to add ears. I added some ears and replaced the original image, producing version 2 (the middle version). Then I thought it would be better with whiskers, so I updated to version 3.

Other users can still go back and look at earlier versions, and preview them in the table, to see how the design has changed over time.

We should use the cat example on the marketing site. http://phacility.com/phabricator/pholio/

I'm worried it may intimidate users who are worried they can not live up to the requisite level of professionalism.