Videos: Just paste the YouTube link. It embeds.
Photos: Upload them first to a host site that works. I usually use this one, though their site seems to be down at the moment: http://imgsafe.org
There may be other image sites that work, but I don't think that Imgur does.
are there any alternative sites that i can use?
Imgur is a well known image hosting site and works well for me.
Follow the Mastering Markdown guide on GitHub for a better idea on how to link to images.