[Feature] Set Image Gallery Thumbnail from one of the existing gallery images

[elshanta]

Is your feature request related to a problem? Please describe.
Currently, it is not possible to change the main Image Gallery thumbnail.

Describe the solution you'd like
It would be great to have the option to select the new gallery thumbnail from one of the existing images inside the gallery.

[peresabcod]

+1
Except those that already have a custom cover, sometimes the first image of a gallery does not represent it in any way (i.e. a barn, a mountain, a random shot in the street, only one performer when there are several, etc..). It would be nice to be able to chose it

[echo6ix]

Related to or duplicate of #863 #1432 #872