To insert a video from Youtube click the video icon above. (3rd from right in bottom line of edit buttons).
In the video dialogue box that pops up add the URL address that Youtube provides when you click the 'share' button.
To insert an image. To insert an image click the image icon in the editor above. (next to icon that looks like an anchor)
In the image dialogue box that pops up click the little icon to the right of the 'Image URL' field.
In the 'file browser' that pops up click the 'upload' button (top left) and browse the image to upload. Click upload and once the image appears in the bottom of file uploader you click it to insert in the image editor box. There you can use the 'appearance' tab to size the image and add description. Most of the site images are 600px so either resize before uploading (Picasa) or do it on the fly in the image editor. You could also follow the theme by adding a border size of '1' and changing the colour code from black to blue. (in the appearance tab).