You do all the tasks you ask about from the buttons above the "edit box" while creating an new message or making a reply. Specifically...
• You upload pictures by using the "Insert Image" button and completing the dialogue displayed. There is a size limit of 1Mb for such files. (See example set to the right!)
• You provide a straight-forward link by using the "Upload a file" option under the "More v" button. There is a 5Mb limit for uploads using this approach. The link below downloads the same file as the image.
• You can display certain videos in-line by clicking the "Embed" button. Then follow the instructions there. The video below appears popular on YouTube at the moment!
If you check the "Message is in HTML Format" box while editing, it is also possible to insert HTML5 code and post video and audio hosted elsewhere. I have done that for this message and everything in the edit box is enclosed in appropriate HTML tags. In the example below the audio controls are provided by the user's browser. They vary slightly from one browser to another, but any modern browser should be capable of providing something that will do the job.