Unfortunately its not uncommon for sites to hotlink others images. Most hosting firms allow one to see some information and thus can be traced back but its rather immaterial as its hard to do anything about. One complains to the hosting firm and they may act or may not.
I'd imagine Auctiva has had many many (many) instances of hotlinking images. I am sure they have a protocol to deal with it if it becomes a big problem such as munching down heaps of bandwidth etc.