Hello MattewMPW,
Regarding to your requirement, you can add it through Signature Editor. To do this, you can follow the steps below:
1. Click File > Options > Mail > Signatures to open the signature editor.
2. Right click on the image in your signature and select Picture.
3. Choose Alt Text tab and type in your alt text.
If you still need further assistance or I misunderstand your meaning, please feel free to let me know.
Regards,
Seven