fb share link with wrong thumbnail

hi again

When I share a picture (opened in lightbox) with fb share button, the image that is shared is the right one, but the thumbnail that is used is always the first picture of the gallery. Strangely it stays the same even after I remove this image completely from the library.

Also another strange issue but probably related is that if you share the first image of the gallery (which is the thumbnail always used), the link opens the next (second) image.

The same thing happens if you refresh the page with the first image open in the lightbox, you are directed to the second image (open in the lightbox)

regards