html5 and the autoplay option

Here is another option to consider if you're willing.

https://community.adobe.com/t5/captivate/so-you-re-looking-to-get-rid-of-that-white-screen-with-play-button-eh/td-p/10780736?page=1

Poster image should work. I have poster images in most of my output files. Have a look at:

http://www.lilybiri.com/published/GameMatchStick/index.html

The workflow I use is described here:

http://blog.lilybiri.com/poster-image-autoplay

That setting no longer affects Auto Play most of the time because most modern browsers will over-ride it.  You can thank the way internet marketers have egregiously abused the option of automatically playing media files on websites for many years and the volume of complaints from web users about having their ears blasted out without warning by media that started playing without their permission.  In response, the browser manufacturers stepped in and made it mandatory for the user to interact with the page before media could begin playing.  That's why this setting does not take effect nowadays.  

If you look carefully on the dialog where you set the AutoPlay option in Captivate you will see a link to the page in the Adobe Captivate HELP system that gives the reasons.