I was testing this myself. So I dropped a webm file size 3.95MB. It appeared to be stalled (on the uploading bar) for a long time, which I expected, because I've heard that uploading these file types takes a long time, perhaps it's interpolating the format or something. Eventually I cancelled the upload bar (and thought the experiment was failed), but left the Roll20 window open. The next day I looked and, sure enough, the webm was playing on the Roll20 tabletop! On the Layer that I had dragged-dropped it onto. What is your Page Size? Try making the Page Size larger in case your graphic appeared beyond the edges. Or try Control-A, the Select All command, and then use arrow keys to see if there's something floated off screen. Check all your layers. Maybe test it again, on an otherwise blank Page. Just some ideas.
