Hi I gather you mean how do you get the watermarked image stored on Auctiva into a listing live on ebay, in which case proceed as follows.
Accessible the image via the images tab -> manage.
Select the image folder it is in and scroll down to the image you want to copy into your live listing.
Against each image a URL is shown typically starting with "http://img.auctiva.com/" and ending with ".jpg", copy the mst of the URL that is from "img.Auctiva" onwards.
Open another browser window, log into your ebay account and goto active listings and against the listing to be modifed select 'revise' from the drop down menu at the right hand side.
Scroll to images, remove the original then select add and then paste the URL from above after the "http://".
Select insert pictures, make any other changes you want to make if any and follow through to submit to complete the revison.
If it is the header or gallery image the change will not be reflected in the Auctiva scrolling gallery or Auctiva storefront without a support request to do so.
I hope that helps.