Como colocar botão de voltar ao topo no blog

1º- Vá em Layout > Adicionar Gadget > HTML/JavaScript

2º- Cole o seguinte código: 

<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.3.2/jquery.min.js"></script>

<script type='text/javascript' language='Javascript'>

var scrolltotop={

//startline: Integer. Number of pixels from top of doc scrollbar is scrolled before showing control

//scrollto: Keyword (Integer, or "Scroll_to_Element_ID"). How far to scroll document up when control is clicked on (1=top).

setting: {startline:100, scrollto: 0, scrollduration:1000, fadeduration:[500, 100]},

controlHTML: '<img src="URL_DA_IMAGEM"/>', //HTML for control, which is auto wrapped in DIV w/ ID="topcontrol"

controlattrs: {offsetx:5, offsety:5}, //offset of control relative to right/ center of window corner

anchorkeyword: '#top', //Enter href value of HTML anchors on the page that should also act as "Scroll Up" links

state: {isvisible:false, shouldvisible:false},

scrollup:function(){

if (!this.cssfixedsupport) //if control is positioned using JavaScript

this.$control.css({opacity:0}) //hide control immediately after clicking it

var dest=isNaN(this.setting.scrollto)? this.setting.scrollto : parseInt(this.setting.scrollto)

if (typeof dest=="string" && jQuery('#'+dest).length==1) //check element set by string exists

dest=jQuery('#'+dest).offset().top

else

dest=0

this.$body.animate({scrollTop: dest}, this.setting.scrollduration);

},

keepfixed:function(){

var $window=jQuery(window)

var controlx=$window.scrollRight() + $window.width() - this.$control.width() - this.controlattrs.offsetx

var controly=$window.scrollTop() + $window.height() - this.$control.height() - this.controlattrs.offsety

this.$control.css({left:controlx+'px', top:controly+'px'})

},

togglecontrol:function(){

var scrolltop=jQuery(window).scrollTop()

if (!this.cssfixedsupport)

this.keepfixed()

this.state.shouldvisible=(scrolltop>=this.setting.startline)? true : false

if (this.state.shouldvisible && !this.state.isvisible){

this.$control.stop().animate({opacity:1}, this.setting.fadeduration[0])

this.state.isvisible=true

}

else if (this.state.shouldvisible==false && this.state.isvisible){

this.$control.stop().animate({opacity:0}, this.setting.fadeduration[1])

this.state.isvisible=false

}

},

init:function(){

jQuery(document).ready(function($){

var mainobj=scrolltotop

var iebrws=document.all

mainobj.cssfixedsupport=!iebrws || iebrws && document.compatMode=="CSS1Compat" && window.XMLHttpRequest //not IE or IE7+ browsers in standards mode

mainobj.$body=(window.opera)? (document.compatMode=="CSS1Compat"? $('html') : $('body')) : $('html,body')

mainobj.$control=$('<div id="topcontrol">'+mainobj.controlHTML+'</div>')

.css({position:mainobj.cssfixedsupport? 'fixed' : 'absolute', bottom:mainobj.controlattrs.offsety, right:mainobj.controlattrs.offsetx, opacity:0, cursor:'pointer'})

.attr({title:'Scroll Back to Top'})

.click(function(){mainobj.scrollup(); return false})

.appendTo('body')

if (document.all && !window.XMLHttpRequest && mainobj.$control.text()!='') //loose check for IE6 and below, plus whether control contains any text

mainobj.$control.css({width:mainobj.$control.width()}) //IE6- seems to require an explicit width on a DIV containing text

mainobj.togglecontrol()

$('a[href="' + mainobj.anchorkeyword +'"]').click(function(){

mainobj.scrollup()

return false

})

$(window).bind('scroll resize', function(e){

mainobj.togglecontrol()

})

})

}

}

scrolltotop.init()

</script>

.............................

A parte em roxo você substitui, pelo URL da imagem. É só hospeda-la em algum site, eu uso o Imgur. 

Espero ter ajudado, beijos ;3

Botões de Voltar ao Topo

Tutorial: Como colocar o botão Voltar ao Topo no seu blog