Dear dutchman48,
Thanks for posting your questions here.
1) Yes, if you enter your images in both the Auctiva Image Selection and Optional eBay Gallery Images sections, your eBay Gallery images will offer the enlarge and zoom features as long as your images are large enough. I believe these features will show up as long as your images are greater than (not just equal to) 500 px on the longest side.
2) No, there is no additional charge for using these features. Adding two or more images to the Gallery section of your listings using the Optional eBay Gallery Images section on the Auctiva lister page is effectively the same as adding those same images direcly through the eBay Sell your Item form.
3) Yes, you can choose to only place your images in the eBay Gallery image section (without placing them in the description) if you would like. In order to do so, you would simply add your images to the Optional eBay Gallery Images section on the Auctiva lister page without placing any images in the Auctiva Image Selection section above.
I hope this helps!
-Mike