The current html output of the embedded video field is a stream of <img> tags inside an <a> tag. This limits the ability to change layout styling.

I propose changing theme_video_cck_video_thumbnail, adding inside the anchor a <div> which would contain the image. For tag class, it would probably be best to have a general class and a provider-specific class, for example this could be something like: class="emfield-video-thumbnail emfield-video-thumbnail-youtube".

In 5.x-1.2 this would mean modifying line 373, to enclose the $output variable in the <div>.

This may be appropriate for other media types in the module, I haven't looked at how they are handled.