Deleted unwanted images

Site: kinetixgroup.com

Im having trouble deleting images from a gallery. I check the box next to the image, select delete image from the drop down menu, click the apply button, click OK on the popup dialog, and the images DO appear to be removed from the list.

However, when I return later to the same gallery the images are always back on the list.

Any clues?

https://www.ads-software.com/extend/plugins/nextgen-gallery/