If you are using webmail for your e-mails you can set your signature up by following these steps:
1. Go to mail.your-domain name in your web browser
2. Log in with your email address and password
3. Go to Settings in the top right corner
4. Go to Identities and click on your e-mail address
5. Fill out your information and add your signature
6. Hit Save
To add an image to your signature in the webmail service, follow these steps:
1. Click on the HTML icon which is for editing HTML source code.
2. In the pop-up HTML Source Editor window, enter in the following code replacing this image path with your own:

3. Click on Update to be dropped back to the signature editing page.
4. Then click on Save to save the signature.
5. Now once this has been done you'll want to click on Settings up at the top-right.
6. From the left-hand menu click on Composing Messages.
7. Then on the Compose HTML messages drop-down menu, select Always, and then click on Save.