It's easy, when you're at the screen where you're typing your message, just under the dropdown menus for the font, there's a row a buttons...you
want the 7th one over, labeled image.
When you click it, a thingie pops up telling you to enter the URL of the picture you want to insert.
To embed a video.....when you start a video...look at your URL bar. At the end of it you'll see a bunch of letters&numbers/symbols....something like
this-------> (GD00YOHw_i4)
Copy that part of the URL and paste it into the space given by the two buttons at the top (vid:youtube/ vid;google)