What is Alt text first of all? It's text used to describe an imagine. Alt text is useful for a number of people, including those that might be using screen reader type of software. According to WHO 2.2 billion people have a near or distance vision impairment, but you can imagine multiple use cases for screen readers - people who might be otherwise occupied and cannot look directly at their device so they have the assistive tech for a particular moment - driving, running, you name it.
I have been trying to add Alt Text consistently across day job and my personal business and realized my Outlook signature was missing one. So had to research how to!
Here is how you can add Alt text to email signatures in Outlook:
1) Open a new email message
2) Select from the top ribbon "Signature" and then click on the option "Signatures..."
3) Select the signature you wish to edit
4) Double click on the photo in your signature and then right click choosing the option "Picture"
5) A new options window will pop-up and you will go to the "Alt Text" tab
6) Write an appropriate description for your signature and then click "Ok"
You are done :)