When you first install Hiri, all emails that you compose will have "Sent from Hiri" as their signature. When you're composing emails you can always edit this signature, but you can also change your signature permanently.
Go to Settings -> Signature and type in your signature. You can also use basic styling options to style your signature.
If you're familiar with HTML, you can edit the HTML of the signature by clicking on the Source button.
To add an image to your signature, you have to do it through HTML.
When you're done editing click Save.