I just added the ability for users of my website to link to mpeg-dash videos hosted on other sites. The videos work great. However, the video/image sharing functionality on my site depends on having access to a thumbnail (still) image of any content that is shared. With mpeg-dash, I don't know of a built-in way to get a still image. I assumed I would find a jpeg representation in the mpd files, but this appears to be merely a proposed addition to the mpeg-dash format that no one is yet using. Note that I have no control over the mpd that I use -- it comes from another site, and I have to use what I'm given.
Is there a built-in way to fetch a thumbnail from a mpeg-dash video? If not, how might I go about making a thumbnail? My back-end language is php, if that matters.