Is it possible to set which image is used for the gallery thumbnail?

I have tried setting a custom order arrangement with the desired image in 1st place but the gallery image reverts to its own random selection.

I have also tried adding just a single image to the gallery in the hope it would use it as the main thumbnail but when I later add more, again it randomises the thumbnail to its own choose.

If it is not currently possible, could you consider including a “Use as main image” option in future releases.