Tidy up of 'Guest' photos which shouldn't be there

During a random search of my site, I came across a 'guest' photo album with multiple (approx 100) photos, many duplicates.. it looks like they came from Facebook... some months ago.. so likely the cause is no longer a problem but my question is... what's the safest way to remove / delete these - can I do it via DB? or do I have to delete them 1 at a time.

I've included the link to where you can see all these guest photos on my site.

I did try the 'Back to Album' link hoping I could then 'delete album' but that produced an 'invalid album' message.

Thanks!