Here is some interesting thing i found, want to share it with you.
Do you want to create like this....
So, Please go through this link : Click Here ( Here just enter your e-mail id in the provided box and it will generate the .png image file for you.
You can keep the same image for your Mail Signatures.
Go to Settings, In "General" section, you will find a column "Signature" ( which will append for all the emails that you are sending) there upload it. It will create a signature with your mail along with domain logo.