To update how your signature reads on your email will depend on where you are sending your email from.   

If you are using a client email (Outlook or Apple's Mail as an example) you would set it up through their system. 

 If you log into your mail  via 

1.You will need to go to settings which is the gear on the lower left hand side.  

2.This will bring a menu up and you can select signature

3. The box in the middle will change with Signature as the header and 2 sub menus mapping or signature

4. Select signature and then click on add 

5. You can now update your signature and add images

6. Click on save of the box you are working on and then on the bigger signature box with the blue heading.  I have added a few images as references.