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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.