To post pics, you'll need some sort of host, unless you upload them directly to the site (under the advanced thread reply option). You could use any number of them, but the more popular sites are Photobucket and Imageshack.
What you're looking for is the tags, which Photobucket provides for you to copy and paste. I'm not sure about Imageshack, since I don't use it. You basically want to see this:
For videos, you'll just need to copy the embed link from somewhere like Youtube. Pay attention, to the little "posting rules" box down below the quick reply box, to see if HTML code is on. If it's not, the embedded video won't work. Good luck!