Yes, you need to keep the photos in your Auctiva folders for as long as the auction or store listings are on eBay. Reason being is that each of your ebay listings contains HTML coding that points towards the images in your Auctiva photo folder each time someone visits the auction or store page, and "grabs" the image. If the image is not in your Auctiva folder, it can't display the photo.
Auctiva provides free image hosting for the images you use in your listings. This can include graphics besides regular old photo images of products. If you are having trouble keeping track of all your images on Auctiva, make folders for them. That way you won't have hundreds of images in just one folder, but rather smaller numbers of images in many folders.
Hope this helps.
P.S. To get images back in your listings, you will need to upload your iamges back on to Auctiva, then call up each of your listings and edit them by putting in the "new" image, and then do a cut-and-paste thing for the HTML coding for editing each listing which you have to do on ebay. Best of luck.