The only thing I know how to do is go into revise on ebay, and delete all the old pics I don't want; come back to Auctiva open up my images, find the one(s) I want and copy the html without the "http://", just the img part of it, then paste it into the add pics on the ebay revision page. It has to be added on the "self hosted" tab. Then if you have selected the old one to be in the ebay gallery, you have to add it there if it doesn't self add it; don't close it because if you want more than one pic added, on the same listing, besides the ebay gallery pic, click add more or whatever it says there and add each one individually using the above method.
If this doesn't answer you question it's because I didn't understand it....and there may be a better way....
this is what I figured out, but I don't think it's necessarily the way others do it. Good luck. and remember I am still a newbie....