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:
You can copy and paste HTML into the HTML Source window:
To add an image to your signature, you have to do it through HTML.
Need a little help creating a HTML signature? There's loads of tools online such as htmlsig.com where you can create your perfect signature.
When you're done editing click OK and then Save.