I realize this question has been answered, but I wanted to share a method to write HTML directly into the signature textarea, to do this:
- Write something in the Signature Rich Text Area provided by Gmail.
- Inspect the text you just wrote with the developer tools for your browser.
- Add any HTML with inline CSS accepted by the email signature tab.
- To get this to validate, you have to change something in the text area to get it updated (if you save without changing anything, nothing will happen).
To get an image in this area, you need to write the HTML and paste it there. You can use the one below to get the results you want:
<td style="width: 80px">
<img src="http://www.example.com/path/to/your/image" width="40" height="40">
<td>Other <strong>HTML</strong> Content</td>