To insert a video, you will need the embedded iframe link. If your video is on YouTube, this is super easy to get. From your YouTube video click "share" then "embed" and grab that iframe link. Once back on Patreon, select the video icon and share the link there.
For images, you will need the direct link of the photo. You can easily get this link by uploading photos to a site like Photobucket or Flickr, then grabbing the direct link from there. Once back on Patreon, select the camera icon and share that link.
Related: How do I add inline images?