F8-Lite Thumbnails- how do you generate using media galleries?

Hi- I’m using the F8-Lite theme (most recent version). I use a gallery in each post and no single images- this causes problems with the thumbnails the site generates, as it only takes text. I’ve tried putting the link to a thumbnail image from the gallery in the custom field value, but it doesn’t work.

Does anyone know of a way to generate a thumbnail from a media gallery (I’m using Grand Flash Album)? Thank you!