Before you try this, realize that you need to go into your blog's HTML template and adjust the code. If you don't know what that is, do not try this. Seriously -- if you don't know what HTML is, if you don't know how to change things in your template already, don't play with this because you can screw your entire blog up very easily if you make a small mistake.
First of all, have your signature ready and in a photo format (preferably .jpg). You could (a) write your own signature, scan it into your computer and save as a photo (b) use a fancy font. That's what I did. I went to this site and downloaded a font. You may need some computer skills to move the downloaded font into your font folder for use.
Then, upload your signature on the internet. I do this with my Picasa Web Album, but you might have another method.
Now, as far as changing the code:
Step 1: Access your HTML template from your dashboard under 'Design'. (I'm not explaining how to get there, as those of you familiar with it will already know how to do it.)
Step 2. Find this code:
Step 3: Insert your image code (see bold) as follows: (you only add ONE line to the code... just the IMG code)
<div class='post-body entry-content'> <data:post.body/> <div style='clear: both;'/> <!-- clear for photos floats --> </div> <img src='http://photobucket.com/signature.jpg'/> <div class='post-footer'>
Step 4: Preview and Save if you're happy with it.