This Blog Tip assumes you already have an image you want to use as your signature, so here goes:
Upload you image (signature) to an image hosting site like Picasa or Photobucket.
If using Picasa: Once your image is uploaded, right-click on it and click "Copy image location"
If using Photobucket: Once your image is uploaded, got to "Share" or click the Blogger icon, then "Get link code" and copy the HTML code for blogs in full (with .jpg, .png or .gif)
Login to your Blogger and go to "Dashboard"
Then click on the "Settings" tab
Click on "Formatting"
Paste the image code into the white box and click "Save Settings"
Now test it! Your signature should appear automatically everytime you write a post.