A colleague who was tasked with ensuring that an organization met Accessibility standards needed to add the alt tag to the logo in a signature:
I need to ensure that a logo in the default e-mail stationary for Outlook complies with Section 508 in the US so the logo MUST have Alt text. I can’t seem to be able to do this in the Signature tools and from all the inaccessible documentation on the Internet (forums) it appears that the only solution is to load a specific stationary each time you want to send a message.I’ve created both a Theme with the “signature” with Alt text and an Outlook e-mail template that has the logo with the Alt Text but can’t find a way to tell Outlook that this is my default. My Theme doesn’t show up in the list of Themes in the Stationary list even though it is stored in the same place as the other Themes.
Since you have a stationery file created, I'll start with the missing theme/stationery problem. When you save stationery in Outlook it needs to be in the Stationery folder under the user's AppData Roaming path at C:\Users\username\AppData\Roaming\Microsoft\Stationery (type or paste this shortcut into the address bar of Explorer and press enter: %appdata%\̴microsoft\̴stationery).
There may be a stationery folder in the user's Documents folder, but Outlook won't use it; stationery needs to be in the Stationery folder under AppData Roaming
After you save or move the html file to the Stationery folder, go to File, Options, Mail, Stationery and Fonts and select the stationery as the default theme.
Add Alt Text in Signature Editor
Outlook will automatically add an alt tag to images in the signature editor, in the format of alt=imagename (without quotes). But you can add a customized alt tag of your choosing, which will include the quotes: alt="This is my logo".
If you need a more robust signature editor, create the signature in a new, blank message form then Select All, Copy (Ctrl+A,C), then Paste (Ctrl+V) into the signature editor. (Don't worry if images appear to be missing. Save it and test it in a new message.)
Or create the signature in your favorite HTML editor, display it in a browser, then Select All, Copy then Paste.