Personally, I think it's best to leave your images here as long as the listing is still available on eBay. That allows the buyer to compare the listing photo to the item they receive (even if the shipping somehow takes a month or whatever) and if a dispute comes up you can still refer to your listing photo (if the photo's simply on your computer, you don't have any way of proving it's the same photo used in the original listing).
That's my two cents. I've never bothered trying to free up space because Auctiva has never said it's necessary. Also, I suppose if it ever became necessary then they could probably set up an auto-purge to their specifications.