Hey Everyone - I know this is quite old but I've just found a workaround in the meantime (you may have found this already).

PLEASE NOTE: DO THIS AT YOUR OWN RISK!!!!

Sammi has a thumbnails folder:

SAMMI LOCATION....\x64\thumbnail

You can edit the thumbnail of your deck, save it and reload sammi.

This will then update the thumnail on your deck.

I'm unsure how quickly this could be programmed up, but at least there's a kind of workaround to it.

this would be nice!