there are callback methods in in attachment_fu, after_attachment_saved maybe you could override it.
you can use rmagick to grab a frame from a movie, but from what i've tried in the past it must go though dump the entrie movie to tmp directory before giving you a single frame.
but look at the attachment_fu.rb source for comments about callbacks and you should see them. and check out the rmagick/imagemagick commands something like this
convert -strip -size 96x96 -resize '96x96>' test.mpg out.jpg
the array index zero supposedly is supposed to grab the first frame only, but still process for a long time leading me to believe its dumping the whole thing to a temp folder.
i dont have this working in a production environment or anything so ymmv, but maybe it helps somewhat.