You can enter any text you want into the editor, including HTML (audio/video embed tags...even script tags), and it will be rendered during preview as-is. So if you are tempted to paste in random javascript you find on the web, think twice.
Post Excerpt
You can configure how much of your post will appear on the blog's index page. The default is to show a "read more" link after the first paragraph. To move the "read more" button somewhere else, paste<!--more-->
wherever you'd like it to appear.
(Note: there's an option to remove the button entirely later on the Download page, as well.)
Images
Upload your image using the LIL blog media upload form, and note the URL it assigns to your image. Use the editor's image button, swapping in the image URL for the placeholder. It should look like this, when you are done:

Videos
Upload your video to the LIL vimeo account, and use this code to embed it, swapping in the correct url.
<div class="embed-container"> <iframe src="https://player.vimeo.com/video/######" width="640" height="480" frameborder="0" webkitallowfullscreen mozallowfullscreen allowfullscreen> </iframe> </div>