Lightship Alley
How do you post video or photos? Ask
They need to be hosted (e.g. saved somewhere online).
Go to that location, copy the url (the bit at the top of the browser). Generally, if you click on a url, it highlights. You can them ricght click and choose Copy or use CTRL+C.
If a photo, click on the icon with a mountain on it. When that opens, just Paste (or CTRL+V) in the source/url box. Repeat as needed.
If a video, click of the icon that looks a bit like a bit of film, and Paste the url in the top box.
Photos will show while you are composing; videos not always. Both should show after posting.
A video:
After posting, I used the Edit function to go back to the video, and goose the display size from 300 to 600 pixels.