Step 1: Click on the HTML icon which is for editing HTML source code.
Step 2: In the pop-up HTML Source Editor window, enter in the following code replacing this image path with your own:
Step 3: Click on Update to be dropped back to the signature editing page.
Step 4: Then click on Save to save the signature.
Step 5: Now once this has been done you’ll want to click on Settings up at the top-right.
Step 6: From the left-hand menu click on Composing Messages.
Step 7: Then on the Compose HTML messages drop-down menu, select Always, and then click on Save.