Set a picture for movie file display in Mac Finder

[Clafoutis]

Hello!

I would like to assign a picture to a movie file "preview".

More precisely, i have some movies in .avi files, and I screen-captured the corresponding synopsis on internet movies sites. It would be nice to view this resumé simply by displaying the avi file in the Finder Coverflow window, instead of seeing whatever first frame the movie starts with, usually some producer's logo.
Ideally, I would like to have an automated script-or-whatever to which I would batch-feed one folder containing a series of subfolders, each subfolder containing exactly ONE .avi file and ONE .jpg file : the script assigning the jpg to the avi (and moving the result into somewhere like a "processed items" folder...)

There is certainly a way to set this first image, but I cannot find it.

Many thanks for helping me!