As stated here https://mail.gnome.org/archives/commits-list/2014-December/msg06079.html and tested personally, after installing media-plugins/gst-plugins-gdkpixbuf shotwell is able to show the video thumbnails.
[master 501df92] media-gfx/shotwell: Version bump, gst-plugins-gdkpixbuf is used (#597968 by Fabio Bonfante). 2 files changed, 63 insertions(+) create mode 100644 media-gfx/shotwell/shotwell-0.24.1.ebuild