First go to your account settings
Click Signature
Click the Upload Button
To insert the image at full size, click Full Image
To insert it at a smaller size click thumbnail and then choose the size you want from the drop down menu
If you would like to preview the size of the signature, click preview, if you are happy with how it looks in the editor, click submit.
Should you choose preview, the editor then will show you both your current signature, and then a preview of your updates. In this case, no updates were made.