How can I get a default image

I am running a query which lists the file and then in the image slider I am filtering on ‘whatever’ this all works fine, however if there is no image to show in the image slider I need it to default to an alternative picture.
Any suggestions on how to do it, I have tried a ‘hide on’ but the problem is the file list always has items.

Example page:
http://testing2019.thegunnery.com.au/4668/guns-and-ammo/shotguns/akkar-3-12g-20inch-3-shot-black-synthetic-extractor-/