Thanks for contributing to our forum. Yes, you can use stock images when listing your items through Auctiva provided that you are posting to a category for which eBay supports that option.
If you are indeed posting an item to a category which supports this option, just click the “Find Product” link that shows up after you have chosen your category and use the tool in the ensuing popup window to select the product you are selling from eBay's product catelog.
Once you have done so, you should see an option called “Include Stock Photo” just above the description editor in the list and you can just check the box next to that option if you would like to include the stock photo in your listing.
I hope this helps. If you need any further assistance with your account, please feel free to contact our Customer Support team by mousing over the “Help” tab and selecting the “File Support Case” option.