Issue with overlay a play button over a youtube thumbnail image

Using Drupal I place videos on my site. Instead of displaying them as embedded video containers, i have a image thumbnail. Now I want to overlay a play button over this youtube thumbnail image, but i get stuck with the css implementation.

please have a look at my code: The second html/css is working, but that's a modified html output and that doesn't get generated from drupal. For some reason I cannot get that reference correct.

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