Ok, I think I got it figured out.
I simply upload the pics to Auctiva, and set them to auto delete in 15 days.
Then I select the listing option to enable eBay's optional image hosting, and uncheck the option for using Auctiva's gallery.
I think that's it. The images then auto delete in 15 days, and I get to use all of Auctiva's great features, without running out of image space.