Fortunately for you, Blogger user Marie Mosley has shared a way for you to automatically add a signature image to each and every one of your blog posts with the help of one CSS rule. Basically, all it would take for you to accomplish this is uploading your signature image to an image or file server and then adding a little CSS code in to your blog template. She even included a small tutorial on how to add a few extra options to your sig.
You can find the detailed steps to her tutorial here.
You can find the detailed steps to her tutorial here.