If you have very long posts or a long line of comments, your visitors find it difficult to surf through your blog. So that's when you need to add a back to top button, and it's configurations are very easy too.
Login To Blogger Go To > Design > Page Elements.
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='https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjG1VxPJ95Vwvm4Cm0iSxCm_JxHm7-9n7voNx5IcTt_eS_DP0G3H0Ikcnp5e4jqGTuJhCTeNZBk3jFfCIYvzXa_e2gzWMwjC1BSh_wOM2BBGAr9IN6Jo_6NWlFYusgYGY6hg02u72oxd1Q/s1600/up+b.png'" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjzCrUaBPjX8ZYF0mu7F4UBLelvrkycErjZPamCKTrIWsvxph14Fq6y-DxkciN7IuaS_XeIaBW5A6YN07rCJlf1TennIGhysRJuScXuhdj34G4RksqKdMdK55FeFa-28pud_tG0kR9MKDI/s1600/up+s.png" onmouseout="this.src='https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjzCrUaBPjX8ZYF0mu7F4UBLelvrkycErjZPamCKTrIWsvxph14Fq6y-DxkciN7IuaS_XeIaBW5A6YN07rCJlf1TennIGhysRJuScXuhdj34G4RksqKdMdK55FeFa-28pud_tG0kR9MKDI/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='https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjG1VxPJ95Vwvm4Cm0iSxCm_JxHm7-9n7voNx5IcTt_eS_DP0G3H0Ikcnp5e4jqGTuJhCTeNZBk3jFfCIYvzXa_e2gzWMwjC1BSh_wOM2BBGAr9IN6Jo_6NWlFYusgYGY6hg02u72oxd1Q/s1600/up+b.png'" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjzCrUaBPjX8ZYF0mu7F4UBLelvrkycErjZPamCKTrIWsvxph14Fq6y-DxkciN7IuaS_XeIaBW5A6YN07rCJlf1TennIGhysRJuScXuhdj34G4RksqKdMdK55FeFa-28pud_tG0kR9MKDI/s1600/up+s.png" onmouseout="this.src='https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjzCrUaBPjX8ZYF0mu7F4UBLelvrkycErjZPamCKTrIWsvxph14Fq6y-DxkciN7IuaS_XeIaBW5A6YN07rCJlf1TennIGhysRJuScXuhdj34G4RksqKdMdK55FeFa-28pud_tG0kR9MKDI/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.