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Simply add a Gadget of HTML/JavaScript type.
add this code in to it:
<!--BACK2TOP-STARTS-->
<a style="display:scroll;position:fixed;bottom:5px;right:5px;" href="#" title="Back to Top"><img onmouseover="this.src='http://1.bp.blogspot.com/_Uewx15poqQo/TKsGcsicHGI/AAAAAAAAAfs/akYoN9GVdZk/s1600/up+b.png'" src="http://4.bp.blogspot.com/_Uewx15poqQo/TKsGdEzrBTI/AAAAAAAAAfw/0KzI0JW2N74/s1600/up+s.png" onmouseout="this.src='http://4.bp.blogspot.com/_Uewx15poqQo/TKsGdEzrBTI/AAAAAAAAAfw/0KzI0JW2N74/s1600/up+s.png'"/></a>
<!--BACK2TOP-STOPS-->
<a style="display:scroll;position:fixed;bottom:5px;right:5px;" href="#" title="Back to Top"><img onmouseover="this.src='http://1.bp.blogspot.com/_Uewx15poqQo/TKsGcsicHGI/AAAAAAAAAfs/akYoN9GVdZk/s1600/up+b.png'" src="http://4.bp.blogspot.com/_Uewx15poqQo/TKsGdEzrBTI/AAAAAAAAAfw/0KzI0JW2N74/s1600/up+s.png" onmouseout="this.src='http://4.bp.blogspot.com/_Uewx15poqQo/TKsGdEzrBTI/AAAAAAAAAfw/0KzI0JW2N74/s1600/up+s.png'"/></a>
<!--BACK2TOP-STOPS-->
you can change the pics if you wish to.The one ending with "up+b.png"(blue) should be replaced with your larger image and the other one(red) with the small image.
Just save your widget and view your blog with a floating Back-To-Top button at the bottom-right corner of your homepage.