... and also be aware Auctiva now charges a minimum of $2.95 per month for their services (it used to be free). 
If the "slide show" you're referring to is the automatically scrolling set of photos of your listed item, that too is available through Auctiva when you choose one of their listing frames.
The promotional scrolling gallery, on the other hand, is different -- it's a showcase of your listed items. There are options for how to have this placed on your listings (top/bottom), options to filter the contents, and there are 3 or 4 different styles, as well as some colour customizing. You can turn it off, but as the earlier poster mentioned, if you stop using Auctiva, the scrolling galleries will be stuck in your listings and need to be removed manually. I'd suggest that if you sign on to Auctiva and then decide to drop it later, before you do, choose the option to not show the scrolling gallery in your listings. This way, it should disappear before you leave Auctiva.
Personally, I've always found the scrolling gallery more effective at the top of a listing, since it can get ignored or missed at the bottom, but others may have a different opinion.