Add an image at ends of WP main navigation.

Not specifically a GPP question so I'm just hoping someone can nudge me in the direction of the best answer.

As a wildlife park website (https://woldswildlife.co.uk) we'd love to have a little image on the left and right ends of the main site navigation (we have a claw mark image we want to use - being wildlife).

I've looked at :before and :after but can’t quite figure out where to do what needs to be done.

Any tips would be appreciated.

Thanks

Martin