This is addressed to a person that is familiar with PHP/Wordpress or to the 'Flexi Pages' plugin developper.
I tested this plugin in case of I can find a way to show page thumbnail (instead of the page link), in plus the usefull options provided by Flexi Pages.
I found this script :
that allow me to list page thumbnail.
I know modify/adjust this block of script, but I need to know where to put it in the file flexi-pages-widget.php to make it work.
And if it's possible, it could be powerfull to implement the function 'get_the_post_thumbnail' in the Admin section of the widget !