Hi Scott,
Yes, this is something you can do within your Auctiva account using the "Replace" option on the Manage Images page. When you check the box next to one of the images in question on that page and click the "Replace" button, you'll be given the option to upload a new image to replace it with.
If you go ahead and do this, the replacement image will then be hosted in your account on the exact same URL as the one you replaced and should be reflected in any associated active listings immediately upon replacement without requiring any further revisions.
I hope this helps. If you need any further assistance with this, please feel free to contact our customer support team using the form on the following page of our site:
http://www.auctiva.com/help/requesthelp.aspx#-Mike