Walkthroughs for custom maps.

How can I simply insert an image here (directly from my computer)? Without uploading it to some service first? When I click "insert image," it asks for the image URL.

In full-screen mode - i.e., in the extended message writing mode, below the message window - there is a "manage attachments" button. Clicking it will open a new small window where you can select and upload the necessary file from your computer, including an image.