Update: 1 and 2 seem to be fixed. 3 still exists though would take a significantly larger effort to troll, though I'm certainly not putting it past anyone.

Three techniques now confirmed

1. object tag

<object data="imageurl"></object>

2. iframe tag

<iframe src="imageurl"></iframe>

3. Story645 confirms vines/tweest can be embeded. Vines autoplay.

Some ideas I've tried that haven't worked:

1. Making a div with a background image through style attribute. The style attribute is parsed out it seems, and divs don't survive.

On a separate note, when I include tags as text (not in pre), subsequent edits interpret the tags as actual html, which is probably a bug that should be resolved.