Hi,
If you mean replace it within an active listing, you would need to edit the image link in the listing to point to the correct new image. Auctiva assigns a unique 9 digit image number for each image it stores in your library, so that's key to fixing an active listing. Look in your image library for the old and new picture and note the number, e.g. ...webimg/123456789_o.jpg where 123456789 is the unique number assigned.
For images embedded in the Description section...
Using the eBay Revise listing, search for the image in the HTML code using the old image number that's assigned by Auctiva and replace each incidence of it's use with the new number (there typically are multiple for each). Use PREVIEW to check result before saving.
For image used for Header and Gallery....
Using eBay Revise listing, remove the current image and replace with link to new image in your Auctiva image library.
Note, replacement of the Gallery/Header will eventually sync back to your Auctiva Store and Scrolling Gallery....but it takes time. If it doesn't happen after 12 hours, open a Support Case and request a sync.
Danno