This is very simple way to add a personal touch to your blog. First make your signature with any photo editing software, I used PaintShopPro. For the image I used png format, so it's transparent.
And then upload it to your web host, I used Picasa Web Album, to get url address.
And then go to Settings > Formating > Post Template
And paste this code to that Post Template box:
<p><image class="centered"alt="post signature" src="Your image url" /></p>
And that's it!