Maybe you have to reembed your video, so that the url changes … since you did not provide a public url I cant see clearly enough how the video is embedded into the iframe.
Then make a screenshot of your starting thumbnail, load that into a webspace or dropbox and paste this embed-code element onto your page (Where your current video is loaded):
The imv-src should be replaced by your handmade thumbnail!!!
When you now click on that image, it should start the video.
If there is a coder out there, that can change that to a normal hyperlink solution - that would be awesome.
Because I can’t guarantee that this will mess up your other img tags on your site…