WP Photo Album Thumbnail different to photo

I’m sorry if this has already been reported but I did not find it when searching.

Whilst testing WP Photo Album we put some sample pictures in. We then deleted them and are putting in some proper ones relevant to the site.
When I put in the 5th photo, I noticed that the thumbnail displayed in the slideshow was completely different to the picture I’d uploaded and was actually from one of the samples.
After looking in /var/www/vhosts/firsdownfayre.co.uk/httpdocs/wp-content/uploads/wppa/thumbs/ the plugin had actually left in thumbnails from the sample pictures with names 5.jpg, 6.jpg etc.

I am not sure if thumbnails are supposed to be deleted when a photo is removed or if they are replaced when you add a new picture and something went wrong when generating a new thumbnail.