The First Way
The first way is to simply add a little bit of code into your Post Template. You can find this by clicking on Settings, then Formatting on your Dashboard. At the bottom of the page, you'll see a field that says Post Template. Simply copy and paste the following code into that box and replace (Your Signature URL) with the url you just pasted into the Notepad doc.

<img src='YOUR SIGNATURE URL HERE'/></a>



The Second Way (The Best Way)
Ok, this is the best way, because if you've had your blog going for a long time, and you're just now getting around to a signature, this will automatically put your signature graphic into all of your previous posts! Yay! Also, it helps your blog load faster.

So, all you need to do is go to your Dashboard and click on Layout, then click on Edit HTML. Make sure to click Expand Widget Templates. Next, hit Ctrl+F and find the following code:

<data:post.body/>
or it might be
<p><data:post.body/></p>


Directly after that code, paste this code:
<p><div align='left'><img src='YOUR SIGNATURE IMAGE URL HERE'/></div></p>


Simply replace the words (Your signature image url here) with the url you copied onto the Notepad doc. and voila, your'e signature will now magically appear on all posts of your blog as well as on every new post you make. To change the graphic, simply put in the new url of your new image. If you want to make your image a button that links to your email address, just add the following code directly before
<a href=mailto:YOUREMAILADDRESS">