The slider prioritizes videos even if you have a thumbnail added, you can force the slider to ignore videos by editing a theme file.
Gigawatt
For the slider:
Open /ocmx/widgets/slider-widget.php and change line 32:
$image = get_obox_image(940, '530', 'thumbnail', '', '');
to
$image = get_obox_image(940, '530', 'thumbnail', "", "", false, true);
For the blog feed:
Open /functions/fetch-list.php and change line 2:
$image = get_obox_image(550, '250', '', 'div', 'post-image');
to
$image = get_obox_image(550, '250', 'thumbnail', "", "", false, true);
Motion Picture
For the slider:
Open /ocmx/widgets/slider-widget.php and change line 35:
$image = get_obox_image(580, 326, 'thumbnail', "", "");
to
$image = get_obox_image(580, 326, 'thumbnail', "", "", false, true);
For the blog feed:
Open /functions/fetch-list.php and change line 2:
$image = get_obox_image(580, '200', '', 'div', 'post-image');
to
$image = get_obox_image(580, '200', 'thumbnail', "", "", false, true);