Yes you can change any element in your email signature including the image.
Recommended Image size: 400x400
Note: Changes happen only for future emails, not those that have already been sent.
To change your image you will need to edit your brand.
How to Edit a Brand:
Account Settings
Select Email Brands
Click the pencil icon next to the brand you want to change.
Hover over your image in your brand, and click the orange trash can.
Once deleted you can replace the image.
