My signature is not set up this way because my blog was professionally designed by someone much smarter & more creative than I am, but I do know that this is how I put my Top Baby Blogs button in all my posts:
If you make your signature somehow, maybe in a picture document or something - Adriel can answer this better than I can. I know she did that part herself. But once you have the file, click on Settings in your blogger dashboard.
Click on Formatting.
Scroll to the very bottom where it says "Post Template."
Then just enter the html code for your file. You may have to upload it to a picture location like I use photobucket. But get the html code & put it in that box. Then it'll show up at the bottom of all your posts.
If that doesn't make sense, let me know. I'm not sure if I'm explaining it clearly!