Firstly, it's no good linking to an image file on your computer: no-one can access it from outside (nor would you want them to). To insert an image, click on the camera icon in the toolbar when composing and entry, and navigate to and select your image, which will then be uploaded.
To add an alias you need to go to http://me.com/icloud - please see
That will set up the alias so that messages to it deliver to your inbox. To send with the alias as the 'From' address in the Mail application on your Mac, go to Mail>Preferences>Accounts, select your iCloud account and in the 'Email address' field add the alias separated from the existing address by a comma. Then it will appear in the drop-down menu when you compose a message.