1) Login your webmail 
2) Go to setting

3)Select Identities -> your email account 

4) Select HTML Signature and you can add your signature at the text editor （can insert text and image ）

5) To add image, click the button

6)Click the button to select the image

7)Click add image button, and it will browse the folder in your pc, select an image to upload.

8)Click save button to save the signature

9) Next, you need to got to setting ->composing message ->Compose HTML message-> Always and click save to save the setting.

10. Then you can now try to send an email with the signature.