Though I've not been using blogspot as my blogging platform for quite a long time already, I'll just create a short howto here on to do just that.
From your blog's admin panel, go to Settings -> Template. From there click on the "Add a Page Element" link, and select "Text" from the selections.
On the editor that pops out, fill up the title, and for the content, click "Edit HTML" and add this lines to the text area:
<a href="http://yourblog.blogspot.com/rss.xml"> <img src="http://www.any-website.com/rss-icon.jpg" />Any text </a>
You can change http://yourblog.blogspot.com/rss.xml to http://yourblog.blogspot.com/atom.xml, whichever that you prefer, and http://www.any-website.com/rss-icon.jpg is a URL to any RSS icon in the net that you prefer (or upload your own)
I hope this helps