1. Log in to your dashboard Design -- Edit HTML

2. Click on "Expand Widget Templates"

3. Search "Ctrl+F" and find the following code:

<data:post.body/>

4.Directly after that code, paste this code:

<p><div align='left'><img src='YOUR SIGNATURE IMAGE URL HERE'/></div></p>

Change the text in red with the link to your image/signature (you can use My Live Signature  to create a signature).
If you want making a clickable signature that links to your email address, just add the following code directly before <img src... but don't leave any spaces.

<a href= "mailto:YOUREMAILADDRESS">

 ...and before the closing div marked with blue add : </a>

 Save the template