Here's how you can change your signature:

go here:
or find Settings:
How To add a signature-screen-shot-2018-11-14-16.56.30.png

go to
Edit Signature >
Option 2 - Upload Image From Your Computer: select file from computer > after selecting file > UPLOAD

once the image is upload. Select [Insert Signature] and then save.

How To add a signature-screen-shot-2018-11-14-17.00.49.png

let me know if you have any questions.