plse note that the “add thumbnail images” is grey. In the setting menu I cannot find the item to check. Only splash tekst is checked

First a note:
My videos all come from a joomla 2.5 platform using JWplayer. In the WP setting tab setup from FV at the bottom is a button to convert from JWplayer, I did not use this because I did not know wat it would do.

But here te requested data:

{"ab":"","ad":"","ad_height":"","ad_width":"","ad_skip":"","align":"","autoplay":"","controlbar":"yes","copy_text":"","embed":"on","end_actions":"","end_action_value":"","height":"1080","hflip":"","lightbox":"","lightbox_caption":"","lightbox_height":"","lightbox_width":"","player_name":"Asmara videos","player_slug":"","playlist":"horizontal","playlist_advance":"","qsel":"","share":"","share_title":"","share_url":"","speed":"","sticky":"","video_ads":"","video_ads_post":"","width":"1920","meta":[{"meta_key":"post_id","meta_value":"1828"}],"videos":[{"caption":"Langeland DK, vissen","end":"","mobile":"","rtmp":"","rtmp_path":"","splash":"","splash_text":"","src":"https:\/\/aeromarine.nl\/wp-content\/uploads\/video import-server\/2017 Vissen Langeland Dk.mp4","src1":"","src2":"","start":"","meta":[]},{"caption":"Shanty kooren Akkrum","end":"","mobile":"","rtmp":"","rtmp_path":"","splash":"","splash_text":"","src":"https:\/\/aeromarine.nl\/wp-content\/uploads\/video import-server\/2017 Shanty kooren.mp4","src1":"","src2":"","start":"","meta":[]},{"caption":"Douchen op voordek","end":"","mobile":"","rtmp":"","rtmp_path":"","splash":"","splash_text":"","src":"https:\/\/aeromarine.nl\/wp-content\/uploads\/video import-server\/2017 Douchen op voordek.mp4","src1":"","src2":"","start":"","meta":[]},{"caption":"Westkust Zweden","end":"","mobile":"","rtmp":"","rtmp_path":"","splash":"","splash_text":"","src":"https:\/\/aeromarine.nl\/wp-content\/uploads\/video import-server\/2016 Baltic Zweden Westkust Deel 1.MP4","src1":"","src2":"","start":"","meta":[]},{"caption":"Op naar Rusland deel 5","end":"","mobile":"","rtmp":"","rtmp_path":"","splash":"","splash_text":"","src":"https:\/\/aeromarine.nl\/wp-content\/uploads\/video import-server\/2015 Baltic Deel 5.mp4","src1":"","src2":"","start":"","meta":[]},{"caption":"Op naar Rusland deel 4","end":"","mobile":"","rtmp":"","rtmp_path":"","splash":"","splash_text":"","src":"https:\/\/aeromarine.nl\/wp-content\/uploads\/video import-server\/2015 Baltic deel 4.mp4","src1":"","src2":"","start":"","meta":[]},{"caption":"Asmara en Zonnewind","end":"","mobile":"","rtmp":"","rtmp_path":"","splash":"","splash_text":"","src":"https:\/\/aeromarine.nl\/wp-content\/uploads\/video import-server\/2015 Asmara en Zonnewind Baltic.MP4","src1":"","src2":"","start":"","meta":[]},{"caption":"Op naar Rusland, deel 3","end":"","mobile":"","rtmp":"","rtmp_path":"","splash":"","splash_text":"","src":"https:\/\/aeromarine.nl\/wp-content\/uploads\/video import-server\/2015 Baltic Deel 3.mp4","src1":"","src2":"","start":"","meta":[]},{"caption":"Op naar Rusland deel 2","end":"","mobile":"","rtmp":"","rtmp_path":"","splash":"","splash_text":"","src":"https:\/\/aeromarine.nl\/wp-content\/uploads\/video import-server\/2015 Baltic Deel 2.mp4","src1":"","src2":"","start":"","meta":[]},{"caption":"Op naar Rusland deel 1","end":"","mobile":"","rtmp":"","rtmp_path":"","splash":"","splash_text":"","src":"https:\/\/aeromarine.nl\/wp-content\/uploads\/video import-server\/2015 Baltic deel 1.mp4","src1":"","src2":"","start":"","meta":[]},{"caption":"Engeland Bud en Oyster ","end":"","mobile":"","rtmp":"","rtmp_path":"","splash":"","splash_text":"","src":"https:\/\/aeromarine.nl\/wp-content\/uploads\/video import-server\/2014-1 Engeland.mp4","src1":"","src2":"","start":"","meta":[]},{"caption":"Engeland Bud en Oyster ","end":"","mobile":"","rtmp":"","rtmp_path":"","splash":"","splash_text":"","src":"https:\/\/aeromarine.nl\/wp-content\/uploads\/video import-server\/2014-2 Engeland.mp4","src1":"","src2":"","start":"","meta":[]},{"caption":"2014 Wakeboarding \"The Kaag\"","end":"","mobile":"","rtmp":"","rtmp_path":"","splash":"","splash_text":"","src":"https:\/\/aeromarine.nl\/wp-content\/uploads\/video import-server\/2014 Wakeboarding Kaag.MP4","src1":"","src2":"","start":"","meta":[]},{"caption":"Rondje Denemarken 3, de Ooszee","end":"","mobile":"","rtmp":"","rtmp_path":"","splash":"","splash_text":"","src":"https:\/\/aeromarine.nl\/wp-content\/uploads\/video import-server\/2013-4 Denemarken deel 3.MP4","src1":"","src2":"","start":"","meta":[]},{"caption":"Rondje Denemarken 2, de Limfjorden","end":"","mobile":"","rtmp":"","rtmp_path":"","splash":"","splash_text":"","src":"https:\/\/aeromarine.nl\/wp-content\/uploads\/video import-server\/2013-3 Denemarken deel 2.MP4","src1":"","src2":"","start":"","meta":[]},{"caption":"Rondje Denemarken 1","end":"","mobile":"","rtmp":"","rtmp_path":"","splash":"","splash_text":"","src":"https:\/\/aeromarine.nl\/wp-content\/uploads\/video import-server\/2013-1 Denemarken Deel 1.mp4","src1":"","src2":"","start":"","meta":[]},{"caption":"Freedom","end":"","mobile":"","rtmp":"","rtmp_path":"","splash":"","splash_text":"","src":"https:\/\/aeromarine.nl\/wp-content\/uploads\/video import-server\/2013-4b Freedom.mp4","src1":"","src2":"","start":"","meta":[]},{"caption":"In memoriam Sailor","end":"","mobile":"","rtmp":"","rtmp_path":"","splash":"","splash_text":"","src":"https:\/\/aeromarine.nl\/wp-content\/uploads\/video import-server\/2013-5_Sailor.mp4","src1":"","src2":"","start":"","meta":[]},{"caption":"To all the Girls. . . . ","end":"","mobile":"","rtmp":"","rtmp_path":"","splash":"","splash_text":"","src":"https:\/\/aeromarine.nl\/wp-content\/uploads\/video import-server\/2013-4A Toallthegirls.MP4","src1":"","src2":"","start":"","meta":[]},{"caption":"Interview Lemmer","end":"","mobile":"","rtmp":"","rtmp_path":"","splash":"","splash_text":"","src":"https:\/\/aeromarine.nl\/wp-content\/uploads\/video import-server\/2012-3 Interview_Lemmer.mp4","src1":"","src2":"","start":"","meta":[]},{"caption":"Einde Martinair Passage","end":"","mobile":"","rtmp":"","rtmp_path":"","splash":"","splash_text":"","src":"https:\/\/aeromarine.nl\/wp-content\/uploads\/video import-server\/2011-4 Martinair_einde_passage.mp4","src1":"","src2":"","start":"","meta":[]},{"caption":"Dagens 2","end":"","mobile":"","rtmp":"","rtmp_path":"","splash":"","splash_text":"","src":"https:\/\/aeromarine.nl\/wp-content\/uploads\/video import-server\/2011-2Dagens2.MP4","src1":"","src2":"","start":"","meta":[]},{"caption":"Texel Vlieland","end":"","mobile":"","rtmp":"","rtmp_path":"","splash":"","splash_text":"","src":"https:\/\/aeromarine.nl\/wp-content\/uploads\/video import-server\/2010_TexelVlieland.mp4","src1":"","src2":"","start":"","meta":[]},{"caption":"Met de Vrijheid naar de Scillys","end":"","mobile":"","rtmp":"","rtmp_path":"","splash":"","splash_text":"","src":"https:\/\/aeromarine.nl\/wp-content\/uploads\/video import-server\/2001 Vrijheid naar Scilly's 2001.mp4","src1":"","src2":"","start":"","meta":[]}]}

Peter

like I said in my first message, in my library no thumbnails are shown on any of my videos. This in spite the plugin I tried “embed video thumbnail”