Solved: You just need to add the word "checked" to the end of the checkbox input lines in modules/boonex/videos/template/form_ghost_template.html:

<input type="checkbox" name="__name_poster__[]" value="{file_id}" onclick="bx_base_general_select_ghost(this, '__name_poster__')" id="__name_poster__-{file_id}" checked /> 

2nd line:

<input type="checkbox" name="__name_thumb__[]" value="{file_id}" onclick="bx_base_general_select_thumb(this)" id="__name_thumb__-{file_id}" checked />

This is just a fail-safe in case the user forgets to check the box before submitting the video.