Thank you for the clarification.
Let me address each point one by one.
Q. "...why is the splash never displayed over the video player itself leaving the latter displaying a black screen ?
This means everytime I use a video player it display an image over the black video player screen. It is usually called a poster and it is pasted over (not above) the video screen. It uses a thumbnail that is one picture of the movie or the first intro picture. You do not have that on any video - why not ?"
A: The splash image is passed in the shortcode:
[wpvp_flowplayer src=http://alex.kby.li/wp-content/uploads/2014/08/clipcanvas_184348_offline.mp4 width=540 height=360 splash=http://alex.kby.li/wp-content/uploads/2014/08/clipcanvas_184348_offline.jpg]
It is automatically generated on video conversion from a video frame. You can specify a number of seconds from which the frame should be generated.
The way video js and flowplayer work is placing the splash image above the video object. This is something that's done by these js libraries.
If you check "Display splash image", it will show a slash image with a button above on video container.
Q: "The splash image which I personally identify as video poster is a standalone picture above the black screen. You have an option that says : "Display splash image". And here is what happens when I check this OFF. It anyway, in any case paste it above the screen. And when I check back in the options I see everytime the check back in that same option."
A: Regarding the black screen, I think I know what the issue was. I changes preload option on video js player from none to auto, so it won't stay as black screen anymore, but rather will preload the 1st frame of the video.
I unchecked "Display splash image" and you can see that the post has preloaded the 1st frame instead: http://alex.kby.li/test-video/
Q: "Why does in the post the splash image displays above the video player when I check : "Display splash image" OFF (and why does it always check itself back ON ?"
A: I cannot replicate this behavior on your site. When I uncheck "Display splash image" and save options, it stays unchecked. If I check it and save options, it stays checked.
Could it be that we both were performing testing on your site at the same time and I checked the option after you unchecked it? : )
Q: "And why am I always made to use flowplayer when I always select video js player ?
The very same here. I select video js and every time in every case it anyway uses the flowplayer."
A: I think the shortcode name might have been confusing here. The shortcode does says "[wpvp_flowplayer]" but it's just the naming that was adopted before we had added the 2nd player option and was not changed, so please pay no attention to it. If you inspect the source code of the page with a video you will see that it is in fact using video js instead of a flowplayer and if you switch to a flowplayer, you will notice that it's a different looking player.
Hope it helps.